Operator battle voices: the official combat lines play by voice slot during battles, one at a time (#73)

Deploy, skill and result lines come from the operators' official voice slots; a newer line takes the channel over (stale callbacks of the replaced line no longer release it), and the result line is spoken by an operator of the player's own battle. Contributed by @Convey123.

docs/SIM.md

@@ -1196,7 +1196,8 @@ Unknown subprofessions fall back to the profession default (test `professions.te
   (`none|arrow|bolt|bomb|lob|orb|drone|enemy|boomerang|droneBomb|chain|chainHeal`; a boomerang's way back has no event — the
   renderer flies it back to the thrower at `BOOMERANG_RETURN_SPEED`; an enemy's `profile.shot` may name another kind,
   e.g. `mortar` for 帝国炮火先兆者, which the renderer does not draw — its fx `bombardShell` is the shell), `['dmg', tgt, amount, type]` (`phys|arts|true|burn|neural|necrosis|apoptosis`),
-  `['heal', tgt, amount]`, `['skill', id, 1|0]`, `['die', id, reason]`, `['leak', id]`, `['status', id, key, 1|0]`,
+  `['heal', tgt, amount]`, `['skill', id, 1|0]`, `['engage', id]` (an ally's first attack that hits an enemy — the client's
+  行动开始 voice, DESIGN §21.30), `['die', id, reason]`, `['leak', id]`, `['status', id, key, 1|0]`,
   `['fx', kind, x, y, extra]` (`hitCap` `{ id, n }`: a leader's hit cancelled by 限伤 — the renderer draws nothing;
   `extra.form` = the unit's model form from then on — an enemy's `content/enemies.js setForm`, a 傀儡师's 替身 — `shared/protocol.js fxForm`),
   `['layer', playerId, bondId, n]` (n = the layers actually added, capped at 999), `['bounty', playerId, coins]`.