Bug: Invalidate cached results whose output assets are missing #111

Summary

After generating a disposable asset and deleting its file, the identical request returned success=true, is_cached=true, and the old asset URL; the runner was called only once. creative_runner.py:211 trusts cache.py:120 without checking output availability.

Environment and evidence

• Review finding: F11 (2026-10-05 product/technical review).
• Baseline: Windows, dev at c1c5dbe. The remote dev matched this commit when filing.
• Evidence: Reproduced.
• Priority recommendation: P1 - proposed first-release blocker. This is review triage, not a production-incident severity declaration.
• No real credentials, paid inference, engine installation, or unrelated process termination were used for review probes. Mocked observations establish the stated code behavior, not live-provider/GPU acceptance.

Reproduction or validation

1. In a disposable asset/cache database, execute a seed=42 request with the engine mocked to return a managed fixture asset.
2. Delete only that fixture's output file.
3. Submit the identical request.
   Observed: success=True, is_cached=True, old asset URL, and only one engine call although the file is absent.

User impact

Users see a successful result that cannot be displayed or exported; R09 and R16 require valid output reuse.

Proposed approach

Validate cache output references and files, invalidate broken entries, and recompute or report an actionable missing-output state. Add startup reconciliation and tests for deleted/corrupt assets.

Acceptance criteria

• A cache hit requires all referenced outputs to remain available and valid.
• Broken entries are invalidated and recomputed or returned as an actionable missing-output state.
• Startup reconciliation detects orphaned references.
• Tests cover deleted/corrupt outputs and valid reuse across restarts.

Verification scope

Real GPU inference, paid-provider compatibility, and a clean-machine packaged desktop journey remain unverified. Any follow-up implementation should target dev under the repository's contribution/branching rules.