ci: bump pinned RELAY conformance validator to v2.0.4

[SoundMatt]

Summary

RELAY tagged v2.0.4 today, fixing #70: go.mod's module path hadn't picked up the /v2 suffix required by Go's semantic import versioning after the v2.0 MAJOR bump, making v2.0.0+ tags uninstallable via go install/go get. cpp-RCP builds RELAY from a local source checkout rather than go install, so it was never actually blocked by that bug — but the relay-conform job's pin (issue #83 / cpp-RCP-15) is deliberately locked to a specific commit for reproducibility, and that pin is now two patch releases stale.

Bumps the "Checkout RELAY (authoritative conformance validator)" step in .github/workflows/ci.yml from:

ref: 3344b3d1a6a4d870d26c81c6f3c3ff5ca2930e16 # v2.0.2

to:

ref: 810d1877362145a407b865426bb007657e42d00f # v2.0.4

Note: 810d187... is the commit the v2.0.4 tag points at, not the tag itself. RELAY's v2.0.4 is an annotated tag, whose own object SHA is f3eeb69d... — that SHA does resolve via git fetch/actions/checkout (GitHub peels it to the underlying commit), but it isn't a commit SHA the way the existing v2.0.2 pin is, so I used the actual commit SHA here to keep the pin consistent with this repo's existing convention (and with what git log/GitHub's UI show when someone inspects the pinned ref directly).

.github/workflows/release.yml and everywhere else in the repo were checked — the RELAY pin only appears in ci.yml.

Verification performed before opening this PR

• Cloned RELAY, checked out 810d1877362145a407b865426bb007657e42d00f, built ./cmd/relay locally — the same way CI's relay-conform job now will.
• Built cpp-rcp's CLI (cmake -B build -DRELAY_BUILD_CLI=ON -G Ninja && cmake --build build --target cpp-rcp).
• relay conform --strict build/cpp-rcp → PASS (all three §12 checks).
• relay interop --strict --protocol RCP build/cpp-rcp → fails, but only because cpp-RCP has never implemented the optional convert subcommand. I confirmed this is pre-existing and unrelated to this bump by running the identical check against the old v2.0.2 pin — same failure. Per RELAY spec §20.1/§20.3, the interop/behavioural-conformance CI gate applies "once convert is implemented" and is only required for the Tooling-conformant tier (convert + subscribe --format json + streaming send --format json); cpp-RCP is Core-conformant only and has never declared convert in its capabilities (cpp-rcp capabilities lists ["version","capabilities","status","send"]). This is a legitimate, known scope boundary, not a conformance gap introduced by this change — non-strict relay interop (matching cpp-RCP's actual capabilities) reports SKIP/PASS.
• Read RELAY's spec/CHANGELOG.md v2.0.1–v2.0.4 entries: all four are tooling/doc/bug-fix patch releases with SpecVersion unchanged at 2.0 (no normative change) — a Go-tooling installability fix (v2.0.4), a goroutine-leak fix in router.Run plus doc corrections (v2.0.3), and two docstring/test-strengthening patches around rcp.ToMessage/FromMessage (v2.0.2, v2.0.1). None require RCP-side canonical-type or wire-format changes in cpp-RCP.
• Ran this repo's full local build/test suite against the bumped pin:
  • cmake --build build --parallel — 213/213 targets built clean.
  • ctest --test-dir build — 52/52 passed.
  • clang-tidy scoped to tests/*.cpp (matching the CI job) — clean, aside from a pre-existing bugprone-infinite-loop false positive in tests/test_request.cpp from a newer local clang-tidy (22.1.8) than CI's pinned clang-14/Ubuntu-22.04 toolchain; confirmed via git diff main --stat that this PR does not touch that file.
  • ASan/UBSan test_regmap job — 132 assertions, 23 test cases, clean.
  • No cpp-fusa binary is available in this local environment to run standalone; the CI-side FuSa gates are unaffected by this change (workflow-only diff).

Why no version bump

This is a CI-workflow-only change with no library code or behavior change, and the pre-existing interop gap is confirmed unrelated to it. Per this repo's convention (e.g. recent fix: commits #90–#95 land within a cycle without bumping CMakeLists.txt's PROJECT_VERSION, while substantive RELAY-spec-driven refactors like c0367d0 do bump patch), cpp-rcp stays at 2.19.0.

Test plan

• relay conform --strict against cpp-rcp built at the new pin — PASS
• relay interop --strict --protocol RCP against cpp-rcp — pre-existing gap confirmed unrelated to this bump (see above)
• Full ctest suite — 52/52 passed
• clang-tidy (tests/*.cpp) — clean modulo unrelated pre-existing local-toolchain false positive
• ASan/UBSan test_regmap — clean
• CI green on this PR (pending)