Harden invariant-suite handoff: retain ancestors and restrict source provenance #211

Summary

The invariant-suite handoff has two confirmed contract gaps:

1. A downstream invariant stage materializes ancestor invariant-suite files into its worktree, but its own artifact publication only captures the current stage's changed/test/provenance paths. The downstream artifact can therefore omit inherited CryticTester, Setup, TargetFunctions, or Properties sources needed by later stages.
2. implementation_paths are accepted from arbitrary repository-relative locations. A malformed or malicious implementation record can copy internal files such as artifacts/, .envrc, or workspace state into the published invariant suite.

Expected behavior

Each invariant-stage artifact should contain the complete, conflict-checked union of all ancestor suite files plus the current stage's authoritative sources. Provenance paths should be restricted to supported source roots (src/, contracts/, and test/) and reject internal state or secret paths.

Acceptance criteria

• Regression tests prove inherited suite files survive setup -> handlers -> coverage -> implementation handoffs.
• Conflicting ancestor files fail closed with an artifact-contract error.
• Regression tests reject internal/secret paths and accept supported source roots.
• Existing source integrity, hard-link, symlink, and size protections remain enforced.