Per plan §3.7.1, the shrinking zone kills bots outside the safe radius.
The engine emits zone_death events (commit f0a0673), but the web viewer
only handled bot_died events, so zone kills weren't visualized correctly.
Changes:
- Add zone_death event collection in drawCombatEffects()
- Visual distinction: yellow-amber lightning bolt marker vs red X for combat
- Zone death animation: fast yellow particles + shockwave
- Screen reader transcript: "Bot X killed by zone"
- Separate summarizeZoneDeaths() for detailed transcripts
Closes: bf-4i44
223bfa3d86