fix(engine): add combat_deaths array to replay JSON top level

Per plan §7.1, the replay format should include combat_deaths array
at the top level for easy access. Previously this field only existed
inside the Result object, causing all replays to show combat_deaths: null
at the top level despite having combat_death events in the turns array.

Changes:
- Add CombatDeaths []int field to Replay struct with json tag
- Populate CombatDeaths in Finalize() from result.CombatDeaths

Closes: bf-36tko

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

engine/replay.go

@@ -20,6 +20,7 @@ type Replay struct {
 	Turns           []ReplayTurn     `json:"turns"`
 	WinProb         []WinProbEntry   `json:"win_prob,omitempty"`
 	CriticalMoments []CriticalMoment `json:"critical_moments,omitempty"`
+	CombatDeaths    []int            `json:"combat_deaths,omitempty"` // bots killed in combat per player (focus-fire)
 }
 
 // ReplayPlayer represents player info in a replay.
@@ -204,6 +205,10 @@ func (rw *ReplayWriter) Finalize(result *MatchResult) {
 	rw.replay.EndTime = time.Now().UTC()
 	rw.replay.Result = result
 	rw.replay.Turns = rw.turns
+	// Copy combat_deaths to top level for easy access (plan §7.1)
+	if result != nil {
+		rw.replay.CombatDeaths = result.CombatDeaths
+	}
 }
 
 // GetReplay returns the completed replay.