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>

scripts/multi_coord_stress/main.go

@@ -117,8 +117,10 @@ type Event struct {
 	// original inbox body (not the LLM-parsed query). Lets us flag
 	// the case where LLM parsing produces a tight-distance match
 	// but the result doesn't actually fit the original ask.
-	// 0 = unrated, 1-5 = judge verdict.
-	JudgeRating       int            `json:"judge_rating,omitempty"`
+	// nil = judge didn't run (or errored); 1-5 = judge verdict.
+	// Pointer (per scrum Opus INFO) so "errored" and "unrated"
+	// don't collide on the int zero value.
+	JudgeRating       *int           `json:"judge_rating,omitempty"`
 	Note              string         `json:"note,omitempty"`
 	TimestampUnixNano int64          `json:"ts_ns"`
 }
@@ -532,7 +534,12 @@ func main() {
 		if len(resp.Results) > 0 {
 			judgeStart := time.Now()
 			rating := judgeInboxResult(hc, *ollama, *judgeModel, ie.Body, resp.Results[0])
-			ev.JudgeRating = rating
+			if rating > 0 {
+				// Only set when we got an actual verdict (1-5); a 0
+				// from judgeInboxResult means decode/HTTP error and
+				// we want absent-from-JSON to flag that distinctly.
+				ev.JudgeRating = &rating
+			}
 			emitSpan("llm.judge_top1", judgeStart,
 				map[string]any{"original_body": ie.Body, "top1_id": resp.Results[0].ID, "top1_corpus": resp.Results[0].Corpus},
 				map[string]any{"rating": rating}, "")
@@ -1389,9 +1396,3 @@ func playbookRecord(hc *http.Client, gw, query, answerID, answerCorpus string, s
 	return nil
 }
 
-func must[T any](v T, err error) T {
-	if err != nil {
-		log.Fatalf("[stress] %v", err)
-	}
-	return v
-}