fix: install relay CLI from /v2 module path, pin to v2.0.4

[SoundMatt]

What

CI's relay-conform job installed the reference RELAY CLI with:

go install github.com/SoundMatt/RELAY/cmd/relay@latest

That bare (pre-v2) module path has been silently resolving to the last
v1.x tag (v1.14.0) ever since RELAY's v2.0 MAJOR bump — Go's semantic
import versioning rules require a /v2 path suffix for a module's v2.x+
tags to resolve at all (SoundMatt/RELAY#70). This repo's relay conform/
relay interop CI gates have therefore been running against a reference
tool three spec releases behind (v1.12, v1.13, v1.14, v2.0 all shipped
since) — and would have kept resolving to v1.14.0 forever, since v2.x
tags now live only under the new module path.

Fixed to:

go install github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4

An explicit version pin rather than @latest, matching this repo's
existing convention of pinning external tool checkouts to a released ref
(e.g. cpp-FuSa is pinned to v0.15.0 in the same workflow, #16).

Conformance verification (done for real, not skipped)

Read RELAY's spec/CHANGELOG.md in full for every version between what
CI was actually running (v1.14.0) and the new pin (v2.0.4): v1.12, v1.13,
v1.14, v2.0, plus the v2.0.1–v2.0.4 patch releases.

• v1.12 — adds "c" as a valid CLI language value. Doesn't affect
  cpp-LIN ("cpp" was already valid).
• v1.13 — deep-audit fix pass. Most fixes are internal to the relay
  tool itself (crossbar routing, router mutex, SOME/IP, stdin bound,
  Ctrl+C handling) or affect other protocols. Two things worth checking
  for LIN specifically:
  • A go:embed glob bug had made spec/vectors/errors/*.json
    unreachable through RELAY's Vector/VectorNames API — meaning
    relay interop's reject-path comparison never actually ran for
    any implementation before this fix, LIN included. This is now fixed,
    so this PR is the first time cpp-LIN's error-handling has been
    checked against relay interop for real. Verified below — it passes.
  • §18.2 gained C++ HealthProvider/MetricsProvider/Drainer binding
    shapes. These are documented as optional interfaces ("applicable
    to all protocols; a C++ implementation exposing them MUST use these
    shapes") — not a new mandatory requirement, so no code change needed
    (cpp-LIN already implements them in this shape regardless).
• v1.14 — §13.7.2 module-name registry expansion for RCP/DDS internal
  naming. Not applicable to LIN.
• v2.0 — BREAKING: replaces RCP's canonical types entirely for TC18
  conformance. RCP-specific, as expected — no LIN impact.
• v2.0.1–v2.0.4 — bug fixes/doc corrections, none touching LIN or C++
  bindings (v2.0.4 itself is the go.mod module-path fix this PR is
  reacting to).

Built RELAY v2.0.4 locally the same way CI now will (go install github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4) and ran the real gates
against a fresh Release build of cpp-lin-cli:

$ relay conform --strict ./build/cli/cpp-lin-cli
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 ./build/cli/cpp-lin-cli
lin-diagnostic-wrong-checksum (LIN)  — cpp-lin-cli EQUIVALENT (correctly rejected)
lin-frame (LIN)                     — cpp-lin-cli EQUIVALENT
lin-id-overflow (LIN)               — cpp-lin-cli EQUIVALENT (correctly rejected)
RESULT: PASS

Both gates pass clean, including the two error vectors that were never
actually exercised by relay interop before v1.13's embed fix. No new
conformance gap — lin::validate_frame() already rejects both cases
(ID > 0x3F, and a diagnostic frame with a non-Classic checksum) with the
correct ErrInvalidFrame sentinel.

Version bump

None. No library code, headers, or test vectors changed — this is a pure
CI-pin fix with no behavior change, same category as #16 (which also
didn't bump the version).

Full local suite (re-run before pushing)

• ctest (Release): 171/171 passed
• ASan+UBSan (Debug, -fsanitize=address,undefined): 171/171 passed
• clang-tidy / gcc-12: not available in this sandbox (macOS, no
  Ubuntu-pinned toolchain); CI runs these natively on ubuntu-22.04
  with the versions the workflow already pins, unaffected by this change.

Test plan

• go install github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4 resolves
  and installs relay 2.0.4 (spec 2.0) — verified locally.
• relay conform --strict against a freshly built cpp-lin-cli — PASS.
• relay interop --strict --protocol LIN against a freshly built
  cpp-lin-cli — PASS, including both LIN error vectors.
• Full ctest suite — 171/171 green.
• ASan+UBSan ctest suite — 171/171 green.
• CI (this PR) — pending.