Interpret #POORBGA display modes (0/1/2)

Context

Found in the BMS spec-compliance audit (finding C-6).

Current behavior

#POORBGA is parsed and stored (bms.poorBga), but every runtime only checks its presence and uses that to disable the #BMP00 default miss image:

• packages/player/src/core/engine.ts (POOR BGA fallback resolution)
• packages/player-tui/src/bga.ts
• packages/player-web/src/scene/lr2/gameplay.ts

The mode value itself (0/1/2) is never interpreted, and no override image is shown — so authoring #POORBGA currently just removes the BMP00 miss image, which also diverges from our own docs/player-spec.md ("used to override the default POOR image").

Expected behavior

Interpret the #POORBGA 0/1/2 miss-layer display modes per the de-facto spec (hitkey command memo), aligned with LR2 behavior where they differ.

Suggested scope

1. Confirm the exact LR2 semantics of modes 0/1/2 (hitkey memo + LR2 verification).
2. Implement the mode switch in the shared POOR-BGA path (core bga-timeline policy + TUI / web compositors).
3. Update docs/player-spec.md / .ja.md and add BGA tests.

Start by confirming mode 0/1/2 semantics in the hitkey command memo and against LR2. Then trace the shared bga-timeline policy and the POOR-BGA paths in packages/player/src/core/engine.ts, packages/player-tui/src/bga.ts, and packages/player-web/src/scene/lr2/gameplay.ts. Done means the modes work across compositors, the player-spec.md and .ja.md docs are updated, and BGA tests cover the behavior.