docs: document all environment variables read by the crate

[nh13]

Summary

Adds docs/environment-variables.md — an exhaustive, audited list of every environment variable the crate reads, grouped by who reads it, with accepted values, defaults, and effects. Links it from the README.

The audit (every env::var/env::var_os/option_env!/env! site across the crate, sub-crates, examples, benches, and tests) found:

• One runtime knob in the production library: PRMI_ISA. Presence-only; enables the byte-identity-safe ISA reseed fast-path in the fused SMEM collector (collect_smems) when a .isa sidecar is present. Read once per process via OnceLock, so it is not on any per-read hot path. Cuts per-reseed SA probes from ~10–20 (cold model launch) to ~1–2 (confirm-only warm start).
• Everything else is dev-only. All other PRMI_*/FIX_* variables are read solely by the benchmark, example, and fixture-generator binaries under examples/, benches/, and prmi-sys/examples/ — they have no effect on a linked consumer.
• CARGO_MANIFEST_DIR, OUT_DIR, CARGO_PKG_VERSION are standard Cargo build-script variables, listed for completeness.

The doc also calls out a naming collision that has caused confusion: the bwa-mem3 consumer's PRMI_ZIGZAG_ISA is a separate, consumer-only variable affecting the consumer's non-fused 'zigzag' finder; to enable this library's ISA reseed on the consumer's default fused path you set PRMI_ISA in the process environment.

Notes

• Docs-only; no library or FFI behavior changes.
• The PRMI_ISA read site in collect.rs was already documented in code; this PR centralizes the full list.

Summary by CodeRabbit

• Documentation
  • Added a new environment variables reference page covering all supported runtime, build, benchmark, example, and test settings.
  • Clarified the production-facing PRMI_ISA variable, including its behavior and default handling.
  • Updated the README to point to the full environment variable list and note which settings affect linked consumers versus local tooling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: fbf68251-4189-4213-9b3f-eee27eaa518d

📥 Commits

Reviewing files that changed from the base of the PR and between 3b9e2c6 and 2dcbb46.

📒 Files selected for processing (2)

• README.md
• docs/environment-variables.md

---

Walkthrough

Adds docs/environment-variables.md (113 lines) documenting PRMI_ISA (production), Cargo build-script variables, and all dev/bench/test harness knobs. README.md gains a 9-line "Environment variables" section summarising PRMI_ISA and linking to the new doc.

Environment Variable Documentation

Layer / File(s)	Summary
PRMI_ISA runtime semantics + README pointer README.md, docs/environment-variables.md	README adds a summary section; doc page details PRMI_ISA presence-only semantics, .isa sidecar fast-path, caching location, and Cargo build-script compile-time variables.
Dev/bench/test harness variables docs/environment-variables.md	Tables covering corpus/input selection, seeding parameters, timing/batching/strategy knobs, criterion/benchmark corpus variables, and make_fixture synthetic generator defaults.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: auditing and documenting all environment variables read by the crate.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/environment-variables

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[nh13]

@coderabbitai review

[coderabbitai]

✅ Action performed

Review finished.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[nh13]

@coderabbitai review

[coderabbitai]

✅ Action performed

Review finished.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.