chore: bump RELAY dependency v1.11.0 -> RELAY/v2 v2.0.4 (v0.12.1)

[SoundMatt]

Summary

RELAY was tagged v2.0.4 today, fixing RELAY#70: go.mod's module path was still github.com/SoundMatt/RELAY after the v2.0 MAJOR bump, which per Go's semantic import versioning rules made v2.0.0+ untaggable/uninstallable via normal go get/go install. It's now github.com/SoundMatt/RELAY/v2.

go-CAN's go.mod was pinned to github.com/SoundMatt/RELAY v1.11.0 — three spec versions (v1.12, v1.13, v1.14) plus the v2.0 MAJOR behind. This PR bumps to the real, current github.com/SoundMatt/RELAY/v2 v2.0.4.

• go.mod/go.sum: require github.com/SoundMatt/RELAY/v2 v2.0.4, old github.com/SoundMatt/RELAY require removed, go mod tidy run.
• Updated all 8 source files' imports from relay "github.com/SoundMatt/RELAY" to relay "github.com/SoundMatt/RELAY/v2": adapt.go, adapt_test.go, can.go, can_vectors_test.go, cmd/cantool/main.go, cmd/cantool/main_test.go, socketcan/bus_linux.go, virtual/bus.go.
• .github/workflows/ci.yml's relay-conform job now installs github.com/SoundMatt/RELAY/v2/cmd/relay@v2.0.4 (was pinned to a stale v1.10.0, itself pre-existing drift from the CI-targeted spec version).

Conformance review (v1.11 → v2.0 is a real multi-version jump, not just a version-string bump)

Read RELAY's spec/CHANGELOG.md entries for v1.12, v1.13, v1.14, and v2.0 end to end:

• v1.12 — adds "c" as a valid CLI language value. Doesn't affect go-CAN (language: "go").
• v1.13 — deep-audit fix pass, but every fix is internal to the relay CLI tool itself (flag parsing, embed globbing, crossbar routing, etc.) or to SOME/IP — nothing an implementation like go-CAN needs to change.
• v1.14 — expands §13.7.2's module-name registry (RCP/DDS-internals naming). Cross-checked go-CAN's actual module directories (virtual, dbc, isotp, j1939, obdii, uds, recorder, safety, socketcan, mock) against every applicable registry entry — all already match exactly, no naming drift.
• v2.0 — BREAKING: replaces RCP's canonical types (Zone/Command/Response/... → TC18 StreamID/ByteBusID/Message/...). RCP-specific; does not touch CAN's canonical types or §8/§15 CAN sections at all.

Verified live, not just inferred: built the RELAY v2.0.4 CLI from source (git checkout v2.0.4 && go build ./cmd/relay/) and ran it against a freshly built go-CAN CLI binary:

relay conform --strict /tmp/go-can    → RESULT: PASS (§12.1/§12.2/§12.3)
relay interop --strict --protocol CAN /tmp/go-can → RESULT: PASS (7/7 vectors EQUIVALENT)
relay compare /tmp/go-can /tmp/go-can → VERDICT: COMPATIBLE

No conformance gaps found — no fixes required beyond the dependency/import bump itself.

Incidental fix

While touching cmd/cantool/main.go anyway, noticed toolVersion was still "0.11.0" despite the already-tagged v0.12.0 release (pre-existing drift, unrelated to the RELAY bump — no test currently gates on it). Corrected to "0.12.1" alongside this bump.

Version bump

Followed this repo's precedent (v0.10.1: "RELAY spec v1.11" + minor fixes) for a dependency-only patch release: v0.12.1. ROADMAP.md updated with the new row.

Test plan

• go build ./...
• go vet ./...
• go test ./... — all packages pass
• go test ./... -race — all packages pass
• gofusa check ./... — clean
• gofusa trace -req-coverage 100 — 100% requirement traceability (94/94), 100% function-annotation density (67/67)
• relay conform --strict / relay interop --strict --protocol CAN / relay compare against the real RELAY v2.0.4 CLI — all pass
• CI (this PR)

golangci-lint run surfaces pre-existing errcheck/staticcheck issues unrelated to this change (unchecked fmt.Fprintln/.Close()/.Send() in tests and examples); it isn't configured or gated in this repo's CI, so left untouched to keep this PR focused on the RELAY bump.