fix: install relay CLI from /v2 module path, pin to v2.0.4 (#50)

CI's `go install github.com/SoundMatt/RELAY/cmd/relay@latest` used the
bare (pre-v2) module path. Since RELAY v2.0.0, Go's semantic import
versioning rules require the /v2 suffix for that path to resolve at
all — the bare path has been silently resolving to the last v1.x tag
(v1.14.0) this whole time, three spec releases behind (v1.12, v1.13,
v1.14, v2.0 all shipped since), and would keep resolving there forever
now that v2.x tags live under the new path (RELAY#70).

Changed to `go install github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4`,
an explicit pin rather than @latest, matching this repo's existing
"pin, don't float" convention for external tool checkouts (e.g.
cpp-FuSa is pinned to v0.15.0 in the same workflow, #16).

Verified conformance against the real RELAY v2.0.4 CLI (built the same
way CI now will) before pushing:
- `relay conform --strict ./build/cli/cpp-lin-cli` — PASS
- `relay interop --strict --protocol LIN ./build/cli/cpp-lin-cli` — PASS,
  including both LIN error vectors (lin-id-overflow,
  lin-diagnostic-wrong-checksum). Those reject-path vectors were never
  actually exercised by any implementation's CI before now — RELAY
  v1.13 fixed a `go:embed` glob bug that had made spec/vectors/errors/*
  unreachable via the Vector API, so `relay interop`'s error-path
  comparison silently only ever ran the happy path. cpp-LIN's
  validate_frame() already rejects both cases correctly, so no gap.

Read RELAY spec/CHANGELOG.md v1.12/v1.13/v1.14/v2.0 in full: v1.12 (add
"c" as a valid CLI language) and v1.14 (RCP/DDS module-name registry)
don't touch LIN or C++. v1.13's §18.2 HealthProvider/MetricsProvider/
Drainer C++ binding shapes are optional-interface documentation, not a
new mandatory requirement — no code change needed. v2.0's RCP
canonical-type replacement is RCP-specific, as expected, and doesn't
touch LIN.

No library code or test vectors changed, and no new conformance gap
was found, so no version bump — consistent with this repo's precedent
for pure CI-pin fixes (#16, which also didn't bump the version).

Full local suite also re-run and green: 171/171 ctest, ASan+UBSan
(171/171). clang-tidy/gcc-12 aren't available on this machine and
weren't run locally; CI runs them natively on Ubuntu with the pinned
versions this workflow already specifies.

Signed-off-by: Matt <47545907+SoundMatt@users.noreply.github.com>

Author: SoundMatt

.github/workflows/ci.yml

@@ -148,7 +148,7 @@ jobs:
           go-version: 'stable'
 
       - name: Install relay tool
-        run: go install github.com/SoundMatt/RELAY/cmd/relay@latest
+        run: go install github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4
 
       - name: RELAY conformance gate
         run: relay conform --strict ./build/cli/cpp-lin-cli