feat: Add real-time kill feed UI

[gpsanant]

Summary

Adds a real-time kill feed UI in the top-right corner of the screen that displays elimination events as they happen.

Server-side changes (game.js)

• Added killFeed array and _addKillEvent() helper to Game class
• Track kill events at both elimination sites: bullet hits (combat) and ring damage
• Each event records killer name, victim name, cause of death, and timestamp
• Include killFeed in broadcast state payload via getState()
• Clear kill feed on startRound() and resetForNextRound()

Client-side changes

• Added kill feed container positioned in top-right corner (index.html)
• Styled entries with color-coded killer/victim names and ring cause
• Process incoming kill events with deduplication via server-assigned event IDs
• Auto-fade and remove entries after 5 seconds (configurable KILL_FEED_DURATION_MS)
• Limit display to last 5 entries (configurable KILL_FEED_MAX_ENTRIES)

Acceptance Criteria Met

1. Kill feed UI visible in top-right corner during gameplay
2. Combat kills show [killer] eliminated [victim]
3. Ring deaths show [victim] was killed by the ring
4. Entries auto-fade after 5 seconds
5. Feed displays last 5 entries, older entries removed
6. NPC bot kills and deaths included
7. Works in both solo and multiplayer scenarios

All 1035 existing tests pass.