feat(engine): compose federated corpus

[tcballard]

Summary

• introduce the central ComposedCorpus with distinct local, retained catalog, and effective projections
• resolve qualified parent canonical IDs and unique unqualified IDs through one source-aware index
• validate canonical-only, same-type, live-Decision-backed overrides without implicit layer precedence
• retain overridden parent history while redirecting only its canonical ID in the effective view
• resolve relationship endpoints, type checks, cycles, scope rows, and inbound counts over source-aware identities
• retain verification-time Markdown bytes by ArtifactKey so consumers never reopen a mutable parent after pin verification

Stack

• Depends on feat(federation): verify materialised parent corpora #455
• Part of corpus-federation: source-aware read model, resolution, routing, and enforcement #270
• Public command activation remains in the next stacked slice

Validation

• composition tests: 7 passed
• source-aware parity tests: 3 passed
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• git diff --check