feat(go-services): Phase 2 — combat_stats accumulator (cross-language exact)

Ports main.py's _combat_session_delta / _combat_merge_into_lifetime (incl. the
documented "offense/defense use latest, additively" quirk) and the combat_stats
handler (session delta -> DB-backed lifetime merge -> delete-then-insert of
combat_stats + combat_stats_sessions). Read handlers gain the live combat
overlay (union live + DB), like Python.

Validation:
- combat.go `combat-merge` CLI folds snapshots through the accumulator; diffed
  against the Python functions on identical input -> byte-IDENTICAL.
- combat_test.go golden test runs in the build (go test now part of the tracker
  Dockerfile).
- Live: 40 combat lifetime rows + 40 session snapshots + rare_events flowing in
  dereth_go via the shadow consumer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go-services/tracker-go/charstats.go

@@ -43,6 +43,14 @@ func (s *Server) handleCharacterStats(w http.ResponseWriter, r *http.Request) {
 // so session is always null. (main.py:1819)
 func (s *Server) handleCombatStatsOne(w http.ResponseWriter, r *http.Request) {
 	cn := r.PathValue("character_name")
+	if s.ingestor != nil {
+		if live, ok := s.ingestor.getCombatStats(cn); ok {
+			writeJSON(w, http.StatusOK, map[string]any{
+				"character_name": cn, "session": live["session"], "lifetime": live["lifetime"],
+			})
+			return
+		}
+	}
 	ctx, cancel := reqCtx(r)
 	defer cancel()
 
@@ -67,13 +75,25 @@ func (s *Server) handleCombatStatsAll(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := reqCtx(r)
 	defer cancel()
 
+	results := make([]map[string]any, 0)
+	seen := map[string]bool{}
+	if s.ingestor != nil { // live overlay first, like Python
+		for char, live := range s.ingestor.allCombatStats() {
+			seen[char] = true
+			results = append(results, map[string]any{
+				"character_name": char, "session": live["session"], "lifetime": live["lifetime"],
+			})
+		}
+	}
 	rows, err := queryRowsAsMaps(ctx, s.pool, `SELECT character_name, stats_data FROM combat_stats`)
 	if err != nil {
 		s.dbErr(w, "combat-stats/all", err)
 		return
 	}
-	results := make([]map[string]any, 0, len(rows))
 	for _, row := range rows {
+		if seen[toStr(row["character_name"])] {
+			continue
+		}
 		results = append(results, map[string]any{
 			"character_name": row["character_name"],
 			"session":        nil,