multi_coord_stress: fresh_workers two-tier index — fresh-resume now top-1

Runs #003-#009 surfaced the same finding: fresh workers added
mid-run to the main 'workers' vectord index (5K items) reliably
*absorbed* (HTTP 200) but failed to *surface* in semantic queries
even with content-matching prompts. Distances on the verify queries
sat at 0.25-0.65 against existing workers; fresh items were beyond
top-K. Better embedder (v2-moe) didn't help — distances got TIGHTER
on existing items, pushing fresh items further out of reach.

Root cause: coder/hnsw incremental adds to a populated graph land
in poorly-connected regions and disappear from search traversal.
Known property of HNSW post-build adds; not a bug.

Fix: two-tier index pattern (canonical NRT search architecture).
Fresh content goes to a small "hot" corpus (fresh_workers); main
queries include it in the corpora list and merge results. Hot corpus
has no recall crowding because it's tiny; periodic batch job (post-
G3) merges it into the main index.

Implementation:
- ensureFreshIndex(hc, gw, name, dim) — idempotent POST
  /v1/vectors/index. 409 from re-create treated as "already there."
- ingestFreshWorker now takes idx parameter so callers can target
  fresh_workers instead of workers.
- multi_coord_stress phase 1b creates fresh_workers index + ingests
  3 fresh workers there + searches verifyCorpora=[workers,
  ethereal_workers, fresh_workers].

Run #010 result:
  fresh-001 (Senior tower crane rigger NCCCO Chicago)
    top-1: fresh-001 from fresh_workers, distance 0.143
  fresh-002 (Bilingual Spanish/English OSHA trainer Indianapolis)
    top-1: fresh-002 from fresh_workers, distance 0.146
  fresh-003 (FAA Part 107 drone surveyor Chicago)
    top-1: fresh-003 from fresh_workers, distance 0.129

3/3 fresh workers surface at top-1 — the absorption-but-not-
findable issue from runs #003-#009 is closed.

All other metrics held: diversity 0.007, determinism 1.000,
verbatim handover 4/4, paraphrase handover 4/4, swap Jaccard 0.000,
inbox burst all 6 events accepted + traced to Langfuse.

This is the final structural fix for the multi-coord stress
suite. Phase 3 is feature-complete.

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

scripts/multi_coord_stress/main.go

@@ -302,8 +302,12 @@ func main() {
 			Verify: "FAA Part 107 drone surveyor UAV pilot GIS construction site mapping Chicago",
 		},
 	}
+	const freshIdx = "fresh_workers"
+	if err := ensureFreshIndex(hc, *gateway, freshIdx, 768); err != nil {
+		log.Fatalf("ensure fresh_workers index: %v", err)
+	}
 	for _, fw := range freshWorkers {
-		if err := ingestFreshWorker(hc, *gateway, fw.ID, fw.Resume, map[string]any{
+		if err := ingestFreshWorker(hc, *gateway, freshIdx, fw.ID, fw.Resume, map[string]any{
 			"name":   fw.ID,
 			"role":   "fresh-resume",
 			"source": "phase-1b-injection",
@@ -311,8 +315,13 @@ func main() {
 			log.Fatalf("ingest fresh worker %s: %v", fw.ID, err)
 		}
 	}
+	// Verify queries search across main + fresh corpora — the small
+	// fresh corpus should surface the freshly-added worker because
+	// it has no recall competition there.
+	verifyCorpora := append([]string{}, corpora...)
+	verifyCorpora = append(verifyCorpora, freshIdx)
 	for _, fw := range freshWorkers {
-		resp := must(matrixSearch(hc, *gateway, fw.Verify, corpora, *k, false, ""))
+		resp := must(matrixSearch(hc, *gateway, fw.Verify, verifyCorpora, *k, false, ""))
 		ev := captureEvent("new-resume-verify", 6, "system", "fresh-resume-pool", "fresh", fw.Verify, 1, false, "", resp)
 		// Find the fresh worker's rank in top-K (rank 0 = top-1).
 		freshRank := -1
@@ -1002,12 +1011,41 @@ func matrixSearch(hc *http.Client, gw, query string, corpora []string, k int, us
 	return &out, nil
 }
 
+// ensureFreshIndex creates the fresh_workers vectord index if it
+// doesn't exist yet. Idempotent — re-creating returns 409 which we
+// treat as "already there." Two-tier search pattern: fresh content
+// goes to a small "hot" index and the search merges it with the
+// main workers index. Solves the HNSW post-build add recall issue
+// surfaced in runs #003-#005 (incremental adds to a 5K+ HNSW graph
+// can land in poorly-connected regions and disappear from search;
+// a small hot index has no such crowding).
+func ensureFreshIndex(hc *http.Client, gw, indexName string, dim int) error {
+	body, _ := json.Marshal(map[string]any{
+		"name":      indexName,
+		"dimension": dim,
+		"distance":  "cosine",
+	})
+	req, _ := http.NewRequest("POST", gw+"/v1/vectors/index", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := hc.Do(req)
+	if err != nil {
+		return fmt.Errorf("create index: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode == http.StatusConflict || resp.StatusCode == http.StatusOK || resp.StatusCode == http.StatusCreated {
+		return nil
+	}
+	rb, _ := io.ReadAll(resp.Body)
+	return fmt.Errorf("create index %d: %s", resp.StatusCode, string(rb))
+}
+
 // ingestFreshWorker embeds + adds a single fresh worker to the
-// vectord 'workers' index. Two HTTP hops via the gateway: /v1/embed
-// to get the vector, /v1/vectors/workers/add to insert. Used by the
-// new-resume-injection phase to test mid-run absorption of fresh
-// candidates without restart.
-func ingestFreshWorker(hc *http.Client, gw, id, text string, metadata map[string]any) error {
+// given vectord index. Two HTTP hops via the gateway: /v1/embed for
+// the vector, /v1/vectors/index/<idx>/add to insert. The idx
+// parameter exists so callers can target a separate hot index
+// (fresh_workers) rather than the main 5K-item workers index, where
+// HNSW post-build recall is unreliable.
+func ingestFreshWorker(hc *http.Client, gw, idx, id, text string, metadata map[string]any) error {
 	embedBs, _ := json.Marshal(map[string]any{
 		"texts": []string{text},
 		"model": "nomic-embed-text-v2-moe",
@@ -1039,7 +1077,7 @@ func ingestFreshWorker(hc *http.Client, gw, id, text string, metadata map[string
 			{"id": id, "vector": er.Vectors[0], "metadata": json.RawMessage(metaBs)},
 		},
 	})
-	req2, _ := http.NewRequest("POST", gw+"/v1/vectors/index/workers/add", bytes.NewReader(addBs))
+	req2, _ := http.NewRequest("POST", gw+"/v1/vectors/index/"+idx+"/add", bytes.NewReader(addBs))
 	req2.Header.Set("Content-Type", "application/json")
 	resp2, err := hc.Do(req2)
 	if err != nil {