Validation report for PIP + mask drafts (overlay, mask, keyframe, missing media)

[renezander030]

Split out of #44, which now covers keyframeable mask geometry only.

A draft built by the PIP plus local mask workflow in discussion #43 has several ways to be silently wrong: the overlay lands on the wrong track, the mask never got attached, the keyframes did not write, or the media the copy points at is missing. None of those fail loudly.

Scope

A validation report over a draft covering overlay count, mask count, keyframe count and missing media count, in the JSON in / JSON out shape the rest of the CLI uses, with -H for the table view.

Check lint first. It already walks materials and reports missing media, so this may be a set of checks inside lint rather than a new command. Prefer extending lint unless there is a concrete reason not to; a new top-level command needs to earn its place.

Depends on

#44 and #77, since the counts only mean something once there is a documented shape to count.

Acceptance criteria

• Counts are correct on a draft with a known overlay, mask and keyframe layout.
• Missing media is reported by path, not just as a count.
• Exit code is non-zero when a check fails, so it is usable in CI. -q stays exit-code-only.
• Test under test/, per CONTRIBUTING.md.