Combat: add Sudden Death phase + fix tiebreak to use absolute HP

Players reported the prior 6-round exhaustion timeout felt arbitrary
when both sides still had plenty of HP. Two fixes:

- Add a "Sudden Death" 4th phase (3-4 extra rounds) so most fights
  resolve naturally before any timeout. Total combat length now caps
  at ~10 rounds.
- When the timeout does fire, tiebreak by absolute HP instead of HP%.
  The %-based logic was unintuitive — a player at 88/123 (71%) would
  lose to a boss at 83/97 (86%) despite having more HP remaining.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/combat_narrative.go

@@ -153,6 +153,8 @@ func phaseHeader(name string) string {
 		return pickRand(decisiveHeaders)
 	case "pre_combat", "pre":
 		return "⚔️ **The fight begins.**"
+	case "Sudden Death", "sudden_death":
+		return "⚔️ **Sudden Death — No Quarter**"
 	case "exhaust":
 		return "⚔️ **Exhaustion — Time Runs Out**"
 	default: