fix: bump CI relay pin to v2.0.4 under the new /v2 module path

[SoundMatt]

Summary

RELAY's conformance-gate job installed the reference relay CLI via a hard pin: go install github.com/SoundMatt/RELAY/cmd/relay@v1.11.0. This was already four minor versions stale (v1.12, v1.13, v1.14 shipped since), and as of RELAY's v2.0 MAJOR bump today, it's no longer installable at all — v2.0 moved the module path to github.com/SoundMatt/RELAY/v2 per Go's semantic import versioning rules (SoundMatt/RELAY#70), so the old import path cannot resolve any v2.x tag.

• Bumped the pin to go install github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4.
• Only one workflow (.github/workflows/ci.yml) referenced the old pin; no local dev docs referenced it (README/CONTRIBUTING already said "RELAY spec v2.0" from an earlier, unrelated audit pass).

Conformance review (v1.11 → v2.0.4 is a real multi-version jump)

Read RELAY's spec/CHANGELOG.md entries for v1.12–v2.0 to check for any newly-applicable general conformance requirements:

• v1.12 — added "c" as a valid CLI language value. Not applicable; rust-LIN already reports "rust".
• v1.13 — fixed two relay CLI bugs (interop with zero binaries, conform --strict flag placement) — CI already passes --strict before the binary path, so unaffected. Also documented HealthProvider/MetricsProvider/Drainer for the C++/Rust bindings — these are optional, capability-declared interfaces (spec §9, "Applicable to all protocols" but explicitly optional). rust-LIN's VirtualBus already implements HealthProvider/MetricsProvider.
• v1.14 — expanded the §13.7.2 module-name registry with RCP-specific module names (avtp, acf, lifecycle, regmap, discovery, request, fragment) and standardized DDS RTPS internal naming. Neither applies to a LIN implementation.
• v2.0 — replaced RCP's canonical types with the real OPEN Alliance TC18 protocol. RCP-specific, breaking change does not touch LIN.

Conclusion: no new conformance gap for LIN from this version jump.

(Note: while auditing §13.7.2 I noticed the registry's virtual (not virtual_bus) module-naming rule already existed at v1.11.0, unchanged since — rust-LIN's module is named virtual_bus. This is a pre-existing structural drift unrelated to this version bump, not caught by relay conform since the spec explicitly documents that conform cannot verify source-level requirements. Flagging for visibility, not fixing here — out of scope for a CI-pin PR.)

Verification performed

Built the reference relay v2.0.4 CLI locally exactly as CI now will, and ran it against a release build of rust-lin:

$ relay conform --strict ./target/release/rust-lin
PASS  §12.1  conforms to RELAY version document schema
PASS  §12.2  conforms to RELAY capabilities document schema
PASS  §12.3  conforms to RELAY status document schema
RESULT: PASS

$ relay interop --strict --protocol LIN ./target/release/rust-lin
lin-diagnostic-wrong-checksum (LIN): relay=EQUIVALENT, rust-lin=EQUIVALENT
lin-frame (LIN): relay=EQUIVALENT, rust-lin=EQUIVALENT
lin-id-overflow (LIN): relay=EQUIVALENT, rust-lin=EQUIVALENT
RESULT: PASS

Also ran the full local build/test suite:

• cargo build --release — clean
• cargo test — 46 unit/integration tests + 2 doctests, all pass
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• rust-FuSa v0.3.10 ungated gates: rsfusa qualify (16/16 cases), rsfusa release (SBOM/provenance/manifest) — both green

No source changes were required, so no crate version bump — this mirrors the precedent set by #11 (the analogous rust-FuSa CI-pin-only fix), which also didn't bump the crate version. Cargo.toml stays at 0.4.2.

Test plan

• relay conform --strict against a locally-built v2.0.4 CLI — PASS
• relay interop --strict --protocol LIN against a locally-built v2.0.4 CLI — PASS (3/3 golden vectors EQUIVALENT)
• cargo build --release, cargo test, cargo clippy -D warnings, cargo fmt --check — all green locally
• rust-FuSa qualify/release gates — green locally
• CI conformance job (.github/workflows/ci.yml) green on this PR