feat(cache): persist verified federated generations

[tcballard]

Summary

• version the persistent read-model layout for source-aware artifact identity, layers, relationship endpoints, liveness, scope rows, and canonical override redirects
• bind every federated cache read to a freshly verified logical generation containing exact child and parent snapshot bytes
• make cold, resident, and stored reads consume the same central ComposedCorpus; a stale or invalid parent returns an error and cannot expose the previous valid model
• include child config/corpus inputs, raw manifest, parent source and pin, overrides, recursion, and stable corpus root in the generation key
• add source/layer/pin provenance to composed validation and relationship findings while keeping inherited warnings and advisories parent-owned
• retain released no-manifest behavior and treat the old store layout as a cache miss

Stack

• depends on feat(engine): activate corpus federation #457
• next: production MCP and federation evaluation

Validation

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check --workspace --all-targets
• focused composition: 8/8
• focused federated cache: 8/8
• git diff --check

All checks pass; one pre-existing intentional differential test remains ignored.