fix: adding a beat could wipe a beat timeline after "Set amount in measure"

[FelipeDefensor]

Summary

"Set amount in measure" puts an arbitrary number of beats in a measure — that is the point of the feature. So beats_in_measure[-1] can legitimately hold more beats than the beat pattern prescribes for that slot.

get_extension_from_beat_pattern treated that as impossible and raised:

ValueError: More beats on starting measure than found in the iterator

and extend_beats_in_measure only recognised an exactly full measure as complete, so it would have grown the over-full measure further rather than starting a new one.

The data loss. Adding a beat in that state raises out of recalculate_measures. TimelineUIs.on_timeline_command recovers by restoring the snapshot it took before the command — but restoring re-runs recalculate_measures, hits the same exception, and aborts after clearing the timeline. The user gets an error dialog and an empty beat timeline; every beat is gone.

Both places now treat a measure at or over its prescribed size as full, so the extension starts a new measure.

Reproducing on dev

1. Beat timeline with beat pattern 4, add 24 beats → beats_in_measure == [4, 4, 4, 4, 4, 4]
2. Select a beat in the 5th measure, "Set amount in measure" → 5 → [4, 4, 4, 4, 5, 3]
3. Delete the last 3 beats, so the over-full measure becomes the last one → [4, 4, 4, 4, 5]
4. Add a beat → "Command failed" dialog, and the timeline is left with 0 beats

With this change step 4 gives [4, 4, 4, 4, 5, 1] and all 22 beats are present.

Test plan

• test_add_beat_when_last_measure_is_fuller_than_beat_pattern — end-to-end through the commands, walks the steps above; raises on dev
• test_starting_measure_fuller_than_beat_pattern — the previously-raising case; get_extension_from_beat_pattern was otherwise untested, so the two neighbouring cases (emptier / exactly as full) are pinned alongside it
• Beat suites green, and the rest of the suite unaffected (the 3 tests/test_app.py failures under pytest -n auto reproduce on dev without this change and pass serially)
• Randomised fuzz over the beat commands (25 seeds x 200 steps of set/reset measure number, set beats in measure, add, delete, distribute): 12 of 25 seeds hit the crash on dev, 0 of 25 with this change

Found by fuzzing while reviewing #480; independent of it.

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