fix(android): report the clip an Android recording really captured - #2566

Android `screenrecord` encodes a frame only when the screen changes, so a window that ends on an
unchanged screen returns a video far shorter than the requested duration, and `record stop` had
nothing to say about it: the reported `durationMs` is host wall clock from `record start` until the
export finished, which is not the length of the file that was just pulled.

`record stop` now measures the pulled MP4 timelines and reports them as `capturedDurationMs`, and
warns with the clip length against the window when the video is two or more seconds short. The
window is measured on the device's own elapsed clock, read before the stop signal and at launch,
because host wall clock drifts against the clock the encoder timestamps frames with; an unreadable
clock or a chunk that answers no duration costs the caller the claim, never the recording.

Measuring the timeline needed an ISO-BMFF box walk, which now lives in
`@agent-device/capture-kit/recording-mp4-duration` and replaces the private top-level atom scan
that MP4 container detection was doing. Stop replay through daemon recovery carries the field too,
so a completion read back from the session resource reports the same numbers it did live.

…ated

The Coverage job's ADR-0019 eager-closure probe failed: `recording/video.ts` evaluated 25 modules on
import where the merge-base evaluated 24, because the MP4 container gate statically imported the box
walk it now shares with the clip-duration read, and `recording/overlay.ts` grew by the same module.
An entry the merge-base already carries gets no growth budget, so the edge moves behind a
function-scoped `await import`: the scan is something recording completion asks for, and importing
this module for `waitForStableFile` or WebM detection should not evaluate a box walker.

The alternative the probe offered -- hosting the walker in a module both growing entries already
evaluate -- would have put an ISO-BMFF walk in `swift-cache.ts` or `video-webm.ts`, or made the
duration read import the Swift validator machinery that sits behind `video.ts`.