fix: keep measures_to_force_display a valid set of in-range indices

[FelipeDefensor]

Summary

Two independent bugs in how BeatTimeline maintains measures_to_force_display. The list is treated everywhere as a set of in-range measure indices, but nothing enforced either property.

1. The same measure could be forced twice. force_display_measure_number appended unconditionally, so setting a measure number to a value it already had left a second entry. unforce_display_measure_number removes a single occurrence, so one "Reset measure number" was no longer enough to hide the label again — and the duplicates were written to the .tla file.

Easy to hit by accident, since the number is applied to every selected measure: re-confirming a number for one measure re-forces all of them.

2. Out-of-range indices survived pruning. reduce_measure_numbers popped from the tail of the list while the last index was out of range. But the list is appended to in the order the user edits measures and is never sorted, so an out-of-range index sitting before an in-range one was left behind. Those stale indices are saved to the file and resurface as spuriously displayed measure numbers once the timeline grows back.

Found while reviewing #480, but independent of it — opened off dev rather than folded into that branch.

Test plan

• test_set_measure_number_twice_forces_display_once — backend, fails on dev with [1, 1] == [1]
• test_reset_measure_number_unforces_display_after_repeated_sets — backend, fails on dev with [1, 1] == []
• test_reduce_measure_numbers_drops_forced_display_out_of_range — backend, fails on dev with [4, 1] == [1]
• test_reset_measure_number_hides_label_set_more_than_once — end-to-end through the commands; fails on dev with '2' == '', i.e. the label stays visible after Reset
• Full suite green (1834 passed, 13 skipped, 2 xfailed, 2 xpassed)
• Randomised fuzz over the beat commands (25 seeds x 200 steps of set/reset measure number, set beats in measure, add, delete, distribute) checking the two invariants: 138 violations before, 0 after

Manual check

1. Beat timeline, several measures, "Measure number periodicity" set so a middle measure is normally hidden.
2. Right-click a beat in that measure, "Set measure number", give it a number — the label appears.
3. Repeat step 2 with the same number.
4. "Reset measure number" — before this change the label stayed visible; it should now disappear.

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