diff --git a/.github/dependabot.yml b/.github/dependabot.yml
new file mode 100644
index 00000000..bc0d0f94
--- /dev/null
+++ b/.github/dependabot.yml
@@ -0,0 +1,64 @@
+# Dependabot version updates. Docs: https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference
+# Every ecosystem carries a 7-day cooldown: no dependency is bumped same-day, so a
+# freshly-published malicious release (the "mini shai-hulud" npm-worm class of
+# supply-chain attack) has a week to be flagged/yanked before we'd auto-PR it.
+version: 2
+updates:
+  # PRIMARY TARGET: actions are SHA-pinned in .github/workflows; Dependabot keeps
+  # those pins current (and re-pins to the new SHA), just delayed 7 days. One
+  # grouped PR keeps the action bumps from flooding the queue.
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7
+    open-pull-requests-limit: 5
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+
+  # Cargo workspace root: core `ordvec` + the `ordvec-python` binding member
+  # (single workspace Cargo.lock). Group minor+patch into one PR; majors stay
+  # separate for manual review.
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7
+    open-pull-requests-limit: 5
+    groups:
+      cargo-minor-patch:
+        update-types:
+          - "minor"
+          - "patch"
+
+  # ordvec-python declared Python deps (numpy). Grouped; no-op if nothing parseable.
+  - package-ecosystem: "pip"
+    directory: "/ordvec-python"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7
+    open-pull-requests-limit: 5
+    groups:
+      pip-minor-patch:
+        update-types:
+          - "minor"
+          - "patch"
+
+  # fuzz/ is a workspace-EXCLUDED standalone crate (own Cargo.lock), nightly-only
+  # cargo-fuzz tooling. Dev-only surface, low priority — group all, tight PR limit.
+  - package-ecosystem: "cargo"
+    directory: "/fuzz"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7
+    open-pull-requests-limit: 2
+    groups:
+      fuzz-deps:
+        patterns:
+          - "*"
diff --git a/.github/workflows/audit.yml b/.github/workflows/audit.yml
index 3e5d2c30..7c51a03c 100644
--- a/.github/workflows/audit.yml
+++ b/.github/workflows/audit.yml
@@ -30,7 +30,10 @@ jobs:
     name: cargo-deny advisories
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: EmbarkStudios/cargo-deny-action@a531616d8ce3b9177443e48a1159bc945a099823 # v2.0.19
         with:
           command: check advisories
diff --git a/.github/workflows/changelog.yml b/.github/workflows/changelog.yml
index a23c505d..c12f325e 100644
--- a/.github/workflows/changelog.yml
+++ b/.github/workflows/changelog.yml
@@ -40,6 +40,9 @@ jobs:
     name: release notes (git-cliff)
     runs-on: ubuntu-latest
     steps:
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
       - name: Gate on a stable SemVer tag
         id: semver
         shell: bash
@@ -55,10 +58,10 @@ jobs:
             echo "::notice::$TAG_NAME is not a stable vMAJOR.MINOR.PATCH tag; skipping release notes."
             echo "ok=false" >> "$GITHUB_OUTPUT"
           fi
-      # actions/checkout v4.3.1, SHA-pinned — a contents:write job pins ALL
+      # actions/checkout v6.0.2, SHA-pinned — a contents:write job pins ALL
       # tooling to immutable refs (no mutable tags), git-cliff-action included.
       - if: steps.semver.outputs.ok == 'true'
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0 # full history + tags for git-cliff
       - name: Generate release notes
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 05f8b7f1..97410f20 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -43,7 +43,12 @@ jobs:
     name: lint (fmt + clippy)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      # Establish an egress baseline (audit-only: logs outbound traffic
+      # without blocking it). A later PR can switch to block + allowlist.
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy, rustfmt
@@ -66,7 +71,10 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: cargo test (default features)
@@ -89,7 +97,10 @@ jobs:
     name: msrv (1.89.0)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@1.89.0
       - uses: Swatinem/rust-cache@v2
       - name: cargo build
@@ -106,7 +117,10 @@ jobs:
     name: deps (no-system-deps + publish dry-run)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: assert no BLAS/faer/ndarray/statrs in the tree
@@ -145,7 +159,10 @@ jobs:
     name: cargo-deny (advisories + licenses + bans + sources)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: EmbarkStudios/cargo-deny-action@v2
         with:
           command: check
@@ -170,7 +187,10 @@ jobs:
       SDE_VERSION: sde-external-10.7.0-2026-02-18-lin
       SDE_URL_BASE: https://downloadmirror.intel.com/913594
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: install Intel SDE
@@ -238,7 +258,10 @@ jobs:
     name: wasm32 (simd128)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
         with:
           targets: wasm32-unknown-unknown, wasm32-wasip1
@@ -279,7 +302,10 @@ jobs:
       matrix:
         os: [ubuntu-latest, ubuntu-24.04-arm]
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: run bench_rank (scaled, seeded synthetic corpus)
diff --git a/.github/workflows/codeql.yml b/.github/workflows/codeql.yml
new file mode 100644
index 00000000..23747032
--- /dev/null
+++ b/.github/workflows/codeql.yml
@@ -0,0 +1,78 @@
+name: codeql
+
+# CodeQL code scanning (GitHub's static-analysis / variant-analysis engine).
+# Scans the Rust core crate, the PyO3 Python bindings, and the GitHub Actions
+# workflows themselves, surfacing security findings as code-scanning alerts.
+#
+# build-mode: none — CodeQL's Rust support and build-less ("no-build") database
+# creation are GA (CodeQL >= 2.23.3). Rust, Python, and `actions` are all
+# analysed WITHOUT a compiler/interpreter run, so no `cargo build` (and no
+# MSRV toolchain) is needed here; the database is built directly from source.
+# This keeps the scan fast and independent of the build matrix.
+#
+# Least-privilege permissions: `security-events: write` is the only elevated
+# scope (required to upload SARIF / publish alerts); `contents: read` +
+# `actions: read` are read-only. Nothing else is granted.
+#
+# Because this workflow runs UNATTENDED on a cron schedule AND holds
+# `security-events: write`, every third-party action is SHA-pinned (immutable)
+# — a compromised mutable tag (@v4/@v6) cannot auto-execute on a scheduled,
+# write-scoped runner. harden-runner audits egress as the first step.
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  schedule:
+    # Weekly. Wednesday 07:42 UTC — deliberately off the audit.yml slot
+    # (Monday 06:17 UTC) so the two scheduled scans never collide.
+    - cron: "42 7 * * 3"
+
+# Least-privilege: read-only at the workflow level; the analyze job elevates to
+# security-events: write (the minimum CodeQL needs to publish alerts).
+permissions:
+  contents: read
+
+concurrency:
+  group: codeql-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  analyze:
+    name: analyze (${{ matrix.language }})
+    runs-on: ubuntu-latest
+    # Skip CodeQL on PRs from forks: a forked PR gets a read-only GITHUB_TOKEN,
+    # so the SARIF upload (needs security-events: write) can't succeed and would
+    # fail noisily. push-to-main + the weekly schedule still scan everything that
+    # lands, and same-repo PRs still get PR-time scanning. (CodeQL is also not a
+    # required status check, so a fork PR is never blocked either way.)
+    if: ${{ github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository }}
+    permissions:
+      security-events: write # upload SARIF / publish code-scanning alerts
+      contents: read # checkout source
+      actions: read # read workflow run metadata (required by CodeQL)
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [rust, python, actions]
+    steps:
+      - name: Harden runner
+        uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@7211b7c8077ea37d8641b6271f6a365a22a5fbfa # v4
+        with:
+          languages: ${{ matrix.language }}
+          # No-build database creation (GA for rust/python/actions).
+          build-mode: none
+
+      - name: Perform CodeQL analysis
+        uses: github/codeql-action/analyze@7211b7c8077ea37d8641b6271f6a365a22a5fbfa # v4
+        with:
+          category: "/language:${{ matrix.language }}"
diff --git a/.github/workflows/python.yml b/.github/workflows/python.yml
index f5d68988..b1ad1574 100644
--- a/.github/workflows/python.yml
+++ b/.github/workflows/python.yml
@@ -36,7 +36,10 @@ jobs:
     name: clippy + fmt (binding)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy, rustfmt
@@ -73,8 +76,11 @@ jobs:
           - { os: macos-latest, python: "3.13" } # macOS arm64 — NEON
           - { os: windows-latest, python: "3.13" } # windows x64
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+      - uses: actions/checkout@v6
+      - uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python }}
       - uses: dtolnay/rust-toolchain@stable