refactor(crate): rename dig-protocol -> dig-peer-protocol (#1383)

[MichaelTaylor3d]

What changed

Package rename ONLY, no behaviour/API change:

• Cargo.toml: name = "dig-protocol" -> "dig-peer-protocol", version 0.2.0 -> 0.2.1 (clears the version-increment gate on a name-only change), repository URL updated to DIG-Network/dig-peer-protocol.
• src/lib.rs: doc header + "consumers depend on X alone" line updated. No dig_protocol:: self-references existed (crate uses crate::), so nothing else in src/ changes.
• README.md / SPEC.md: crate-name references and the dig_protocol::{...} import example updated to dig_peer_protocol. Every chia-protocol/chia_protocol reference and the DIG_MESSAGE = 220 text are untouched.
• CHANGELOG.md: new [0.2.1] entry documenting the rename.
• .github/workflows/publish.yml: fixed a pre-existing copy-paste bug in the create-release job — it hardcoded "dig-gossip" (release name, body heading/description, install snippet, README link) instead of this crate's name. Now says dig-peer-protocol. Secret names untouched.

How verified

• cargo build --no-default-features: PASS (aws-lc-sys/native-tls/rustls features fail to build in this checkout for an unrelated reason — a Windows MAX_PATH limit hit by aws-lc-sys's CMake TryCompile scratch dir under this deeply-nested worktree path; reproduced identically on main before the rename via git stash, confirming it's a pre-existing environment issue, not caused by this change).
• cargo test --no-default-features: 47/47 tests PASS (dig_message, dig_message_type, introducer_wire — decode/serde/error/round-trip coverage all green).
• cargo fmt --check: clean.
• cargo clippy --no-default-features -- -D warnings: clean.
• Stray-reference sweep: grep -rn "\bdig-protocol\b\|\bdig_protocol\b" . --include=*.rs --include=*.toml --include=*.md --include=*.yml (excluding target/.gitnexus) returns only the intentional CHANGELOG rename note.

Follow-up (not in this PR)

• Consumers dig-gossip and dig-slashing migrate their dependency name to dig-peer-protocol in separate follow-up PRs.
• This crate publishes to crates.io as a NEW package dig-peer-protocol 0.2.1 (orchestrator-owned); the old dig-protocol crates.io package is yanked once consumers have moved. Repo rename on GitHub is also orchestrator-owned.

Refs #1383