fix(player): shorten the default POOR BGA display window to 500ms (LR2)

[nulltask]

Closes #140.

Real LR2 ships <poorbga>500</poorbga> in its config.xml, and its changelog documents the miss-BGA display time as "adjustable in ms (default: 500ms)". Our shared default (DEFAULT_POOR_BGA_DISPLAY_SECONDS) was 2 s — four times longer — and both the TUI compositor and the web LR2 scene inherit it.

Changes

• DEFAULT_POOR_BGA_DISPLAY_SECONDS: 2 → 0.5 (packages/player/src/core/bga-timeline.ts), with the LR2 provenance recorded in the doc comment.
• New regression test pinning the boundary exactly: the overlay is visible at 0.49 s and gone at 0.5 s under the strict < expiry. The previous suite passed identically with either 2 s or 0.5 s, so nothing pinned the value.
• Moved one existing sample point (0.5 s → 0.3 s) that would otherwise sit exactly on the new expiry boundary.
• Fixed a stale "2-second POOR-judgement window" comment in the web LR2 scene's renderBga doc.
• Documented the 500 ms default in docs/player-spec.md / .ja.md (the POOR section previously stated no duration).
• Changeset: patch for @be-music/player, @be-music/player-tui, @be-music/player-web.

Notes

• LR2 exposes this as a per-user setting; exposing it here is left as the optional follow-up the issue mentions — this PR fixes the default divergence only.
• The beatoraja web scene intentionally does not use this constant (it holds the miss layer until the next non-POOR judge, as beatoraja skins expect); the constant's comment now says so.

Testing

• vitest run packages/player-tui/src/bga.test.ts packages/player/src/core/bga-timeline.test.ts — 35 passed (includes the new boundary test).
• tsc typecheck for @be-music/player, @be-music/player-tui, @be-music/player-web; oxlint clean.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	be-music-player-demo	b3fa635	Commit Preview URL Branch Preview URL	Aug 19 2026, 11:06 AM

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Exports Benchmark

• Base SHA: 6c325bc55fa9
• Head SHA: b3fa63509b8d
• Comparable cases: 242
• Regression threshold: 8.00%
• Base runs: median of 3
• Head runs: median of 3

Summary

Metric	Value
Improved (>= threshold)	1
Regressed (<= -threshold)	1
Unchanged	240
Median change	-0.73%
Mean change	-0.77%
Head benchmarked cases	242
Head skipped cases	21

Top Regressions

API	Base ops/s	Head ops/s	Change
audio-renderer.writeAudioFile	4880.76	4141.89	-15.14%

Top Improvements

API	Base ops/s	Head ops/s	Change
lr2-skin.informationTypeToPlayVariant	2019777.61	2317155.67	+14.72%