feat: preserve opaque symlinks in cache payloads

[MarcoPorcellato]

Summary

• Preserve ordinary Unix symbolic links as opaque payload leaves during bounded cache inventory, clone fallback, reuse, promotion, and cleanup.
• Keep the control plane link-free and permit payload mode only below exact validated cache-key data roots.
• Serialize inventory through an existing non-writing entry lock and strengthen staging ownership, recovery, failure injection, and operation-level tests.
• Document the trust boundary, platform evidence, testing contract, ADR, approved design, implementation plan, and changelog entry.

Trust claim checklist

• Public claims in this pull request do not imply:
  • native execution where only contract-level qualification is available,
  • signed identity or full execution attestation when only A0 evidence is present,
  • zero-cost operation or equivalent absolute cost reductions.
• I described any source, runtime, workflow, permission, secret, policy, schema, or dependency impact below.
• Roadmap text is presented as planned work, not as implemented or qualified behavior.

Impact and rollback

• Runtime or trust-boundary impact: validated Unix payload descendants may be opaque links; control-plane and payload-root links still fail closed. Inventory acquires the existing entry lock without changing its bytes or metadata. The documented residual risk remains limited to a non-cooperative local actor that bypasses the advisory lock.
• Dependency or supply-chain impact: none; Cargo.toml, Cargo.lock, schemas, and public API remain unchanged.
• Data, secret, permission, or network impact: no new receipt fields, secrets, permissions, network calls, or public evidence fields.
• Rollback path: revert this pull request. No migration or schema rollback is required.

Evidence checklist

• I listed the exact focused checks run for this diff.
• I ran cargo fmt --all -- --check because Rust files changed.
• I ran the relevant contract tests and recorded intentionally not-run gates.
• git diff --check origin/main..HEAD passes.
• User-visible changes update CHANGELOG.md.
• New or changed links point to existing authoritative targets.

Validation notes

Exact qualified candidate: 72c496082b5851e529acddc972a61951b9098cbf on Darwin arm64 through the pinned Docker-compatible runtime.

• CCP generation: 3
• Configuration digest: sha256:bc348df299ee58ecc8f7cfc7f2dc743b5c03f1a90f7e5dbff45754017955e901
• Receipt SHA-256: 20533d762d13ba3777a2004f2600ba63c61925df0cc044a954d6c161829b34c2
• Receipt ID: sha256:c0d616bf4b22192181a1f9a33bf98bc91a5b611e2a9033c07e138c4e1d58956a
• Source snapshot manifest: sha256:bb2014bc3b521a7a04c5f5598930e13305d5694939a7e3a23af58e44ea45ebcf across 213 tracked entries
• format: PASS
• test --locked --all-targets --all-features: PASS
• clippy --locked --all-targets --all-features -- -D warnings: PASS
• doc --locked --no-deps: PASS
• Release metadata check: PASS
• Exact-commit receipt integrity and trusted-plan policy verification: PASS
• Evidence branch: ccp-evidence/72c496082b5851e529acddc972a61951b9098cbf

This qualification proves the exact source, reviewed plan, pinned runtime, required checks, and receipt integrity. It does not claim Windows-native execution, GitHub-hosted project execution, cryptographic producer identity, candidate installation, or stable-producer replacement.