Support extended visible note channels 1A-1Z / 2A-2Z as playable notes

Context

Found in the BMS spec-compliance audit. Per hitkey's command memo, visible notes span #xxx11-1Z (1P) / #xxx21-2Z (2P); the alphabetic tail is used by keyboard-mania-style and other extended-lane charts.

Current behavior

Only 11-19 / 21-29 are extracted as playable notes — packages/player/src/playable-notes.ts accepts lane characters 1-9 only. The 24 KEY SP / 48 KEY DP lane-mode estimation and key bindings exist in packages/player/src/core/lane-layout.ts, but the extended channels themselves never become score/judge targets, so such charts play silent and judgeless.

This is currently a documented limitation (docs/player-spec.md BMS support matrix), so this issue is the tracking item for lifting it.

Suggested scope

1. Extend the playable-channel classification (@be-music/chart channel predicates + playable-notes.ts) to the alphabetic lane range.
2. Wire the extracted notes into the existing 24/48-key lane layouts and input bindings.
3. Cover with extraction + judging tests and update the support matrix in docs/player-spec.md / .ja.md.

Start with packages/player/src/playable-notes.ts and the @be-music/chart channel predicates, then inspect packages/player/src/core/lane-layout.ts for the existing 24/48-key layouts and input bindings. Done means alphabetic channels become playable score and judge targets, extraction and judging tests cover them, and the support matrix is updated in docs/player-spec.md and .ja.md.