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/main.go

@@ -44,6 +44,14 @@ type Server struct {
 }
 
 func main() {
+	// `tracker-go combat-merge` reads a JSON array of cumulative session
+	// snapshots from stdin and prints the folded lifetime — a deterministic hook
+	// for cross-language parity testing against the Python combat functions.
+	if len(os.Args) > 1 && os.Args[1] == "combat-merge" {
+		runCombatMergeCLI()
+		return
+	}
+
 	logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelInfo}))
 	slog.SetDefault(logger)