feat(engine): activate corpus federation

[tcballard]

Summary

Activates the accepted corpus-federation decisions on top of #456.

• loads one verified child/parent composition before public reads
• routes validate, relationships validation, resolve/find/diagnose, decisions-for, grounding, Sentry, and gate through the shared effective model
• preserves source/layer/pin and complete override provenance while keeping parent history qualified-addressable
• evaluates inherited scope and code constraints against child code
• keeps parent warnings/review advisories parent-owned
• bounds all mutation walks to the local writable layer and rejects direct parent-materialisation targets
• preserves the released no-manifest paths

This is draft PR 4 in the federation stack. Profile guidance, export composition, persistent serving, MCP activation, and the evaluation certification remain in later stacked PRs.

Validation

• cargo test -p asdecided-core — 102 unit tests plus integration/doc tests passed (1 intentional ignored differential)
• cargo clippy -p asdecided-core -p decided --all-targets -- -D warnings — passed
• cargo check -p decided — passed
• activation-owned federation black-box contracts — 11/11 passed
• git diff --check — passed

Refs #270