fix(studio): a slow thumbnail job keeps a reader that other clips still decode from - #6643

Summary

Follow-up to #6629, from Genesis's review. A thumbnail job that timed out judged its reader by its own progress only: whether it had waited for the open, or had decoded a frame itself. Take a poster job that got an already-open reader while a sibling clip was decoding on it, and then needed more than its budget to reach its one frame (a slow link, or Chrome's six connections held). It logged a timeout and retired a healthy reader, so the next clip re-read the index (at a 3.4 GB offset for a long camera file).

Now the pooled reader counts the frames that every job decodes from it (progress.framesFromAnyJob). A timed-out job retires the reader only if no frame came from it while that job held it. A job that waited for the open still counts that open as progress, as before.

No visible change

The reader's frame count in thumbnailSourcePool.ts and thumbnailVideoDecoder.ts: 17 lines, no .tsx, .css or .html. A thumbnail looks the same; a slow clip no longer makes the next one re-read its source.

What I measured

• thumbnailVideoDecoder.test.ts + thumbnailScheduler.test.ts: 65 pass, including the new keeps the reader when a clip times out while another clip decodes frames from it.
• The existing drops a reader for one clip's timeout only once the clip still decoding on it ends still passes. Its sibling has decoded no frame when the timeout fires, so the reader is still retired.
• fails without the fix: when each job gets its own counter (progress = { framesFromAnyJob: 0 } per acquire, the old per-job rule), the new test fails: the timeout is logged and the next clip reopens the source.
• Pre-commit hooks (lint, format, fallow, typecheck) pass.

What I did NOT exercise

• A real slow link in the browser. This is the decision rule, tested with mediabunny mocks.
• Genesis suggested counting completed reads as well as decoded frames. I count frames only: a frame is the result every job is waiting for, and counting reads would mean threading a counter through mediaFetch.

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