POOR BGA default display time should be 500ms, not 2000ms (LR2)

[nulltask]

Context

Found by reverse-engineering the actual LR2 (LR2body.exe, beta3, 2010-02-01) shipped files during the LR2-alignment work.

First-party evidence

• LR2files/Config/config.xml ships <poorbga>500</poorbga> as the default.
• LR2 changelog history.txt (entry dated 081xxx): "コンフィグにミスBGAの表示時間変更を追加しました。ms単位で変更が可能です。(デフォルト;500ms)" — "Added a config option for the miss-BGA display time, adjustable in ms (default: 500ms)."

So real LR2 shows the POOR / miss BGA layer for 500ms by default (user-configurable).

Current behavior

DEFAULT_POOR_BGA_DISPLAY_SECONDS = 2 in packages/player/src/core/bga-timeline.ts — i.e. 2000ms, 4× longer than LR2. This constant is shared by the TUI compositor and the web LR2 scene, so both diverge.

Fix

• Change DEFAULT_POOR_BGA_DISPLAY_SECONDS to 0.5.
• Update the regression coverage and the doc note in docs/player-spec.md / .ja.md.
• Optionally expose it as a setting later (LR2 makes it configurable), but the default is the divergence.