test(media): cover package acceptance and realistic rejection

[alphastorm]

Scope

• Keep 12 package-level behavioral tests: canonical acceptance, realistic corruption, private metadata, stale provenance, and the reproduced missing-manifest-record regression.
• Add only an invocation-local repository/process/probe context for isolated fixtures. Production CLI defaults remain real.
• Make the manifest type guard reject shapes for which it records errors, preventing the reproduced undefined asset-record dereference.
• No standalone parser test suite, obscure format matrices, parser changes, media changes, exclusions, or coverage-threshold changes.

Proof

• Full clean-checkout suite: 608 pass, 0 fail, 3,972 assertions.
• Local full-suite LCOV under unchanged Codecov ignore rules: 6,569 / 9,655 lines = 68.04% (previous reported coverage 61.03%, pre-drop 65.49%). Hosted Codecov result will be checked separately.
• Focused media checker coverage: 87.45%; existing contract module: 84.71%, through package-level behavior.
• Retained malformed-manifest regression fails when the original guard bug is restored in an isolated temporary module; passes with the fix.
• Actual media:check, scoped strict TypeScript check, repository check, capability leak scan, and identifier leak scan pass.
• Only CHANGELOG.md and the checker/test files changed. Temporary coverage and mutation files removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.96%. Comparing base (39c99a2) to head (9c2292e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
+ Coverage   61.79%   67.96%   +6.17%     
==========================================
  Files          35       35              
  Lines        9681     9654      -27     
==========================================
+ Hits         5982     6561     +579     
+ Misses       3699     3093     -606     

Files with missing lines	Coverage Δ
scripts/media/check-readme-media.ts	87.45% <100.00%> (+75.27%)	⬆️

... and 2 files with indirect coverage changes

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