fix(mcp): re-check type-override rationales on every version-1 request [roadmap:deterministic-substrate]

[tcballard]

Summary

This closes the known limitation recorded in #495.

A type override's rationale Decision may resolve outside the captured version-1 snapshot: #495 made root-owned rationales resolve across the root's whole working tree. The version-1 FederatedCacheTracker warm path skips recomposition while the captured generation is unchanged. As a result, a cached decided-mcp kept serving after that Decision was retired, while a fresh process correctly failed with corpus-federation-invalid-override.

While any type override is applied, the version-1 tracker now recomposes on every request, which is what the version-2 tracker already does for every request. Corpora without type overrides keep the warm path unchanged.

Version 2 had no gap: GraphFederatedCacheTracker::read_or_recompose runs compose_verified_federation, including the rationale check, on every request.

Roadmap / ADR Trace

• Roadmap: decisions/roadmaps/deterministic-substrate.md (Tranche C)
• ADR-150 (type-override rationales), ADR-112 (freshness floor), ADR-148 (serving generation keys)

Scope

• rust/rac-engine/src/derived_cache.rs: one condition on the v1 warm path.
• rust/decided-mcp/tests/spec_registry_freshness.rs: regression test.
• docs/validation.md, CHANGELOG.md.

Product / Architecture Decisions

• Recomposing per request while overrides are applied was chosen over plumbing the resolved rationale file into the generation's watched files. The resolved path is only known after composition, but the generation is captured before it, so watching it would need a second, post-build freshness input. Type overrides are rare, and the cost matches what every version-2 server already pays.

User-Facing Contract

• A retired or removed rationale Decision now fails the next cached request (corpus-federation-invalid-override … not a live Decision) instead of being noticed only after a restart.
• No other behaviour change. The warm path is untouched for corpora without type overrides.

Verification

• cargo fmt --check; cargo clippy --workspace --release --no-deps -- -D warnings and --all-targets: clean; cargo test --workspace --release: pass.
• Mutation check: without the fix, the new test's third get_summary succeeds (the stale model is served), so the test fails.
• Conformance 11/11; live-corpus invariants PASS; corpus gates exit 0.

Review Path

1. derived_cache.rs (8 lines).
2. The test.

Notes For Reviewer

• docs(artifacts): state that bundle synonyms aid classification only [roadmap:deterministic-substrate] #498 also adds a CHANGELOG bullet at the same place, so the second of the two to merge needs a trivial CHANGELOG resolution.

Implementation Process

Implemented with AI assistance under the roadmap contract; final scope, review, and acceptance decisions were made by the maintainer.