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.

shared/protocol.js

@@ -341,7 +341,7 @@ export function validateC2S(msg) {
 
 // Event tuple kinds inside `b.ev` (DESIGN §8.2).
 export const EV = Object.freeze({
-  SPAWN: 'spawn', ATK: 'atk', DMG: 'dmg', HEAL: 'heal', SKILL: 'skill', DIE: 'die', LEAK: 'leak',
+  SPAWN: 'spawn', ATK: 'atk', DMG: 'dmg', HEAL: 'heal', SKILL: 'skill', ENGAGE: 'engage', DIE: 'die', LEAK: 'leak',
   STATUS: 'status', FX: 'fx', LAYER: 'layer', BOUNTY: 'bounty', DEPLOY: 'deploy',
 });