scrum multi_coord_phase3: 4 fixes from cross-lineage review

Cross-lineage scrum on bundle 87cbd10..f971e64 (3,652 lines)
produced 4 actionable findings, all defensive hardening.

1. (Opus WARN) internal/langfuse/client.go:queue
   Synchronous Flush at maxBatch threshold blocked the calling
   goroutine for the full 5s HTTP timeout when Langfuse hiccupped,
   defeating the "best-effort, never blocks calling path" contract
   in the package doc. Now fire-and-forget via goroutine.

2. (Opus + Kimi convergent) cmd/observerd/main.go:handleInbox
   - Free-form priority string was accepted; "nonsense" passed
     through unchecked. Now closed enum: urgent|high|medium|low (+
     empty defaults to medium). Tested: TestInbox_RejectsBadPriority.
   - No size cap on body, only emptiness check; multi-MB payloads
     would bloat observer's ring + JSONL. Now 8 KiB cap returns 413.
     Tested: TestInbox_RejectsOversizedBody.
   - Subject/sender/tag concatenated into InputSummary without
     newline stripping; embedded \n could corrupt JSONL line-based
     parsers. New sanitizeInboxField strips \r\n + caps at 256 chars
     before interpolation.

3. (Opus INFO) scripts/multi_coord_stress/main.go
   Removed dead `must[T]` generic — tracedSearch took over the
   fail-fast role for matrix searches, so the helper became unused.

4. (Opus INFO) scripts/multi_coord_stress/main.go:Event
   `JudgeRating int` collapsed "judge errored" and "judge said
   unrated" both to 0. Changed to *int — nil = errored, 1-5 =
   verdict. judgeInboxResult still returns 0 on error; caller
   gates on > 0 before assigning.

Dismissed (with rationale):
- Opus WARN ExcludeIDs ordering: verified by code read — filter
  applies after sort + before top-K truncation as documented;
  no slot waste possible.
- Opus INFO 10 prior-run reports contradict #011: those are
  point-in-time snapshots; intentional history.
- Kimi INFO Langfuse error suppression: design intent (best-effort
  per package doc).
- Kimi INFO contract schema validation: defer until contract count
  grows enough to make hand-edit drift a real risk.
- Kimi INFO paraphrase prompt duplicated across lift + multi_coord:
  defer (lift to internal/paraphrase/ when a third consumer appears).
- Qwen HOLD: single-line, no actionable finding.

go test ./cmd/observerd ./internal/langfuse all green; multi_coord
driver builds clean.

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

cmd/observerd/main.go

@@ -110,6 +110,41 @@ type inboxMessage struct {
 	Tag      string `json:"tag,omitempty"`
 }
 
+// validInboxPriorities is the closed set of priority values. Per
+// scrum (Kimi WARN): a free-form priority string was accepted, so
+// downstream consumers parsing on exact enum could see "nonsense"
+// values. Now rejected at the boundary.
+var validInboxPriorities = map[string]bool{
+	"":       true, // empty defaults to medium below
+	"urgent": true,
+	"high":   true,
+	"medium": true,
+	"low":    true,
+}
+
+// inboxMaxBodyChars caps the inbox body at a sane size so a large
+// payload doesn't bloat observer's ring buffer or JSONL log. Per
+// scrum (Opus WARN): only emptiness was checked, so a multi-MB body
+// would be accepted unconditionally.
+const inboxMaxBodyChars = 8 * 1024
+
+// inboxMaxFieldChars caps subject/sender/tag at a much tighter size.
+// These appear inside the InputSummary string + JSONL log entry, so
+// long values bloat every record and embedded newlines corrupt
+// downstream line-based parsers.
+const inboxMaxFieldChars = 256
+
+func sanitizeInboxField(s string) string {
+	// Strip newlines so multi-line attacker-controlled strings don't
+	// corrupt the JSONL log (one-line-per-op invariant).
+	s = strings.ReplaceAll(s, "\n", " ")
+	s = strings.ReplaceAll(s, "\r", " ")
+	if len(s) > inboxMaxFieldChars {
+		s = s[:inboxMaxFieldChars]
+	}
+	return s
+}
+
 func (h *handlers) handleInbox(w http.ResponseWriter, r *http.Request) {
 	var msg inboxMessage
 	if !decodeJSON(w, r, &msg) {
@@ -123,12 +158,25 @@ func (h *handlers) handleInbox(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "body required", http.StatusBadRequest)
 		return
 	}
+	if len(msg.Body) > inboxMaxBodyChars {
+		http.Error(w, fmt.Sprintf("body exceeds %d chars", inboxMaxBodyChars), http.StatusRequestEntityTooLarge)
+		return
+	}
+	if !validInboxPriorities[msg.Priority] {
+		http.Error(w, "priority must be urgent|high|medium|low", http.StatusBadRequest)
+		return
+	}
 	if msg.Priority == "" {
 		msg.Priority = "medium"
 	}
+	// Sanitize the fields that go into the InputSummary string so
+	// newlines don't corrupt the JSONL log.
+	sender := sanitizeInboxField(msg.Sender)
+	subject := sanitizeInboxField(msg.Subject)
+	tag := sanitizeInboxField(msg.Tag)
 	op := observer.ObservedOp{
 		Endpoint:      "/observer/inbox/" + msg.Type,
-		InputSummary:  fmt.Sprintf("from=%s priority=%s tag=%s subject=%s", msg.Sender, msg.Priority, msg.Tag, msg.Subject),
+		InputSummary:  fmt.Sprintf("from=%s priority=%s tag=%s subject=%s", sender, msg.Priority, tag, subject),
 		OutputSummary: msg.Body,
 		Source:        observer.SourceInbox,
 		Success:       true,

cmd/observerd/main_test.go

@@ -212,6 +212,35 @@ func TestInbox_RejectsEmptyBody(t *testing.T) {
 	}
 }
 
+// TestInbox_RejectsBadPriority locks the priority enum (per
+// scrum Kimi WARN — free-form priority strings were accepted).
+func TestInbox_RejectsBadPriority(t *testing.T) {
+	r := newTestRouter(t)
+	body := []byte(`{"type":"email","sender":"x","body":"y","priority":"nonsense"}`)
+	req := httptest.NewRequest("POST", "/observer/inbox", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400 on bad priority, got %d", w.Code)
+	}
+}
+
+// TestInbox_RejectsOversizedBody locks the body-size cap (per scrum
+// Opus WARN — only emptiness was checked before).
+func TestInbox_RejectsOversizedBody(t *testing.T) {
+	r := newTestRouter(t)
+	bigBody := strings.Repeat("x", inboxMaxBodyChars+1)
+	body := []byte(`{"type":"email","sender":"x","body":"` + bigBody + `","priority":"high"}`)
+	req := httptest.NewRequest("POST", "/observer/inbox", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+	if w.Code != http.StatusRequestEntityTooLarge {
+		t.Errorf("expected 413 on oversized body, got %d", w.Code)
+	}
+}
+
 // TestWorkflowRun_UnknownMode locks the 400 path on workflow definitions
 // that reference modes not registered with the runner. The harness's
 // reality test runs depend on this so an unknown-mode misconfiguration