Battle BGM follows the round: rounds 1–7 无畏者 (bat_kazimierz2_2), rounds 8–13 骑士之日 (bat_kazimierz2_1), as in the official level data (#72)

联防 keeps the escaped stage's corrosion track (#110). Contributed by @Convey123.

docs/ASSETS.md

@@ -84,7 +84,7 @@ The research JSONs in `docs/research/` (03, 05, 07) define **which** ids are nee
 | Token Spine | fexli: the default model, or else the first skin variant (`spine/{tokenId}/{variant}/Spine/`) | `spine/token/{tokenId}/{stem}.*` |
 | Enemy Spine (PC build, premultiplied alpha) | isHarryh/Ark-Models `models_enemies/{key}/`, file names from `models_data.json` | `spine/enemy/{enemyId}/{stem}.*` |
 | Enemy Spine that no dump carries (灼热源石虫 / 炽焰源石虫) | the local client only (`tools/local-extract/extract.py ENEMY_SPINES`, optional); never downloaded and never required: an overlay of the web alias (`enemies[id].spineLocal`) | `local/spine/enemy/{enemyId}/{stem}.*` (listed in `data/local-assets.json`) |
-| BGM | AA2 `voice` branch `audio/sound_beta_2/music/**` | `audio/bgm/{file}.mp3` |
+| BGM | AA2 `voice` branch `audio/sound_beta_2/music/**` (大厅/休整期 `act1autochess`, 开战 `act13side/m_bat_kazimierz2_{1,2}` — 骑士之日 / 无畏者; the 开战 track follows the round: `_2` 无畏者 rounds 1–7, `_1` 骑士之日 from round 8) | `audio/bgm/{file}.mp3` |
 | SFX (UI, battle, per unit) | AA2 `voice` `audio/sound_beta_2/**`, mapped from `audio_data.json` banks | `audio/sfx/{same sub-path}.mp3` |
 | Fonts: Bender Regular and Light, Novecento Wide | TimWangZi/The-font-of-Arknights | `public/fonts/*.{otf,ttf,woff2}`, `public/fonts/fonts.css` |
 
@@ -153,11 +153,13 @@ All paths are URL paths relative to the site root, for example `/assets/char/ava
                                     // group + name of the same picture (the client takes the local one first)
   prof:    { icon: {caster…warrior}, large: {…}, battlecard: {…, token}, sub: {[subProfessionId]: url} },
   audio: {
-    bgm:     { lobby, prep, combat, unite?: { intro?, loop }, boss: { intro?, loop } },
-             // intro then crossfade to loop (1 s); `unite` = 联防's own track — the official
-             // escaped_single / escaped_multi levels declare `bgmEvent = corrosion` (卡西米尔 act13d5d0),
-             // so the rescue phase does not reuse the 作战's track (audio.js bgmKeyFor 'unite', falling
-             // back to `bgm.combat` for a manifest that lacks it)
+    bgm:     { lobby, prep, combat, combatAlts?: [{ intro?, loop }, …], unite?: { intro?, loop }, boss: { intro?, loop } },
+             // intro then crossfade to loop (1 s); combatAlts = the 开战 tracks of the mode's own act, chosen by the
+             // round, not drawn: [0] = m_bat_kazimierz2_1 骑士之日 (rounds 8–13), [1] = m_bat_kazimierz2_2 无畏者
+             // (rounds 1–7) — audio.js combatTrackFor / bgmKeyFor 'combat:<i>', the same on every client;
+             // `unite` = 联防's own track — the official escaped_single / escaped_multi levels declare
+             // `bgmEvent = corrosion` (卡西米尔 act13d5d0), so the rescue phase does not reuse the 作战's track
+             // (audio.js bgmKeyFor 'unite', falling back to `bgm.combat` for a manifest that lacks it)
     bossBgm: { [bossId]: { intro?, loop } },                   // per-boss track of its R14/R15 level
     sfx: {
       ui:     { click, back, confirm, tab, pick, drop, error, buy, sell, income, refresh, freeze, levelup,