docs: mandate spec document's own version literals match spec/version.json (§19.5)

[SoundMatt]

Summary

Closes #144 (NEW-SPEC-6). Adds §19.5 "This document's own version
literals" and the CI mechanism that enforces it: the spec document's
own text must never drift from spec/version.json the way its
"0.1" example literals once did (fixed by hand in v2.2.3).

Background

Requirement 14 (§17) already obligates an implementation's declared
spec_version to trace back to spec/version.json rather than a
hand-copied literal kept in sync by memory. This repository's own
spec document was never held to the same standard for its own text —
§12.1/§12.2's JSON examples and §13.5's Docker LABEL example drifted
to "0.1", three major versions stale, before being manually fixed in
v2.2.3. That fix closed the immediate symptom but not the underlying
gap: nothing stopped it from recurring.

What's added

• §19.5: normative MUST that every version literal in this
  document's own text — the "spec_version" examples (§12.1, §12.2,
  §12.4, §17.2, §20.6), the LABEL io.relay.spec-version example
  (§13.5), and §19.4's "Current version" line and Go/C++/Rust snippets
  — matches spec/version.json, with this repository's own CI
  enforcing it.
• TestSpecVersionMatchesVersionJSON and
  TestSpecDocumentVersionLiteralsMatchVersionJSON
  (version_consistency_test.go): read spec/version.json and
  spec/relay-spec.md via the package's own embedded evidence
  (evidence.go) and fail the build on any drift across all 12
  checked literals (5 named anchors + 7 spec_version example
  occurrences).
• New REQ-RELAY-100, tagged in evidence.go where the two embedded
  artifacts (spec/version.json, spec/relay-spec.md) originate.
• Not a relay conform check and no new §17 Requirement.
  relay conform verifies binaries, not the spec document that
  defines it — this is a repository-CI-verified discipline alongside
  Requirements 13/14, with no change to implementation-facing
  obligations.

Version

PATCH (v2.7 → v2.7.1) — doc addition, no normative change to existing
conformant implementations. SpecVersion unchanged.

Verification

• go build/vet/test clean, gofmt -l . clean, golangci-lint run 0
  issues
• gofusa check 0 errors/PASS, gofusa trace -req-coverage 100
  100%/100% (100/100 requirements traced)
• Mutation-tested: bumped spec/version.json to a throwaway value and
  confirmed both tests failed with a precise per-literal diff for all
  12 checked literals; separately reworded one anchor's surrounding
  prose and confirmed the test reports "pattern not found" rather than
  silently passing (guards against the anchors going stale
  themselves). Both restored, re-verified green.