feat(mcp): serve verified federated corpora

[tcballard]

Summary

• activate the central source-aware composition across remaining CLI read surfaces, local-subject diagnostics, Herald, and stdin proposal validation
• serve all six MCP tools from one freshly verified logical generation with cache-on/cache-off parity, qualified parent history, canonical override redirects, bounded provenance, and stale-parent refusal
• fail closed when federation topology, child config, manifest shape, source, or pin changes; audit preparation failures exactly once with no stale returned identity
• use the central composed resolver for both exact MCP tools, source-distinct ambiguity paths, canonical-only qualified references, and effective-vs-qualified-history graph projections
• retain source-aware identities through graph traversal, audit returned records, relationship findings, response budgets, summaries, and local Git recency without assigning child history to inherited artifacts
• extend the existing DecisionGrounding evaluation with a 40-artifact parent, lexical hard negatives, and a high-inbound graph decoy while preserving the v0.28 graph-floor invariant
• keep no-manifest repositories on the released single-corpus paths

Stack

• depends on feat(cache): persist verified federated generations #458
• next: inherited export composition and Portal identity

Closes #270

Validation

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• MCP: 18 unit tests and 39 integration tests, including 8 federation and 14 HTTP tests
• composition: 9/9
• federated cache: 10/10
• decided eval --check federation track
• decided validate decisions/: 462 valid, 0 invalid, 12 skipped
• decided relationships decisions/ --validate: 2,931 checked, 0 issues
• decided gate decisions/: 0 blocking, 134 existing advisories
• git diff --check

All listed checks pass; one intentional differential test remains ignored.