matrix: split boost / inject thresholds — kills Shape B cross-pollination

Run #003 surfaced rampant cross-pollination: Q2's "OSHA-30 forklift
Wisconsin" recording (w-4435) became warm top-1 for Q19 (dental
hygienist), Q20 (RN), Q21 (software engineer), and 6 other unrelated
staffing queries. Cause: InjectPlaybookMisses inherited the same
DefaultPlaybookMaxDistance (0.5) as the boost path, but inject is
structurally riskier than boost — boost only re-ranks results that
already retrieved on their own merits, while inject FORCES a result
into top-K, so a loose match cross-pollinates wrong-domain answers.

Empirical motivation from v3:
  Implied playbook hit distances for cross-pollinated cases: 0.20-0.46
  Implied distances for the 6/6 paraphrase recoveries:        0.23-0.30
  Threshold of 0.20 should keep most paraphrases, kill the OOD bleed.

Implementation:
- New DefaultPlaybookMaxInjectDistance = 0.20 in playbook.go.
- New PlaybookMaxInjectDistance field on SearchRequest (override).
- InjectPlaybookMisses signature gains maxInjectDist param; hits whose
  Distance exceeds it are skipped (boost path may still re-rank them).
- TestInjectPlaybookMisses_RespectsInjectThreshold locks the contract
  with one tight + one loose hit, asserting only the tight one injects.
- Existing tests pass explicit threshold (0 = default for tight tests,
  0.5 for the dedupe test which uses 0.30 hits).

Run #004 result on identical queries with the split threshold:

  Verbatim discovery        8 (vs v3's 6 — judge variance, separate)
  Verbatim lift             6 / 8 (75%)
  Paraphrase top-1          6 / 8 (75%)
  Paraphrase any-rank in K  6 / 8

OOD queries Q19/Q20/Q21 ALL show warm top-1 = cold top-1 (no
injection) — cross-pollination eliminated where it was wrong-direction.
Mean Δ top-1 distance dropped from -0.164 (v3, distorted) to -0.071
(v4, comparable to v1's -0.053).

Two paraphrases missed in v4 (Q9, Q15) were ones where qwen2.5
rephrased liberally enough to drift past 0.20 — Q9: "Inventory
specialist..." → "Individual needed for inventory management..." and
Q15: "Engaged warehouse associate..." → "Warehouse associate currently
engaged with a robust history...". The system correctly refusing to
inject when it's not confident is the right product behavior; the
boost path still re-ranks recorded answers when they appear in regular
retrieval.

The Q6 ↔ Q7 cross-pollination ("Forklift-certified loader" ↔
"Hazmat warehouse worker") is legitimate — these are genuinely similar
staffing queries and the judge ranks both directions as plausible.

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

internal/matrix/retrieve.go

@@ -53,8 +53,14 @@ type Result struct {
 //   PlaybookCorpus: index name; empty = DefaultPlaybookCorpus.
 //   PlaybookTopK: number of similar past queries to consider; 0 =
 //     DefaultPlaybookTopK.
-//   PlaybookMaxDistance: cosine ceiling for "similar enough"; 0 =
-//     DefaultPlaybookMaxDistance.
+//   PlaybookMaxDistance: cosine ceiling for "similar enough" on the
+//     BOOST path (re-rank in place); 0 = DefaultPlaybookMaxDistance.
+//   PlaybookMaxInjectDistance: tighter cosine ceiling for the SHAPE B
+//     INJECT path; 0 = DefaultPlaybookMaxInjectDistance. Splitting the
+//     two thresholds is intentional — boost is safe at loose thresholds
+//     because it only re-ranks results that already retrieved on their
+//     own merits, while inject forces results in and so cross-pollinates
+//     wrong-domain answers if the threshold is too loose.
 //
 // Metadata filter (post-retrieval structured gate):
 //   MetadataFilter: map of metadata-field → expected value. Results
@@ -76,8 +82,9 @@ type SearchRequest struct {
 	UsePlaybook         bool           `json:"use_playbook,omitempty"`
 	PlaybookCorpus      string         `json:"playbook_corpus,omitempty"`
 	PlaybookTopK        int            `json:"playbook_top_k,omitempty"`
-	PlaybookMaxDistance float64        `json:"playbook_max_distance,omitempty"`
-	MetadataFilter      map[string]any `json:"metadata_filter,omitempty"`
+	PlaybookMaxDistance       float64        `json:"playbook_max_distance,omitempty"`
+	PlaybookMaxInjectDistance float64        `json:"playbook_max_inject_distance,omitempty"`
+	MetadataFilter            map[string]any `json:"metadata_filter,omitempty"`
 }
 
 // SearchResponse wraps the merged results plus per-corpus return
@@ -233,8 +240,12 @@ func (r *Retriever) Search(ctx context.Context, req SearchRequest) (*SearchRespo
 			slog.Warn("matrix: playbook lookup failed; skipping boost+inject", "err", err)
 		} else if len(hits) > 0 {
 			resp.PlaybookBoosted = ApplyPlaybookBoost(resp.Results, hits)
+			maxInjectDist := float32(req.PlaybookMaxInjectDistance)
+			if maxInjectDist <= 0 {
+				maxInjectDist = float32(DefaultPlaybookMaxInjectDistance)
+			}
 			var injected int
-			resp.Results, injected = InjectPlaybookMisses(resp.Results, hits)
+			resp.Results, injected = InjectPlaybookMisses(resp.Results, hits, maxInjectDist)
 			resp.PlaybookInjected = injected
 			if injected > 0 {
 				// Re-sort + truncate after injection. ApplyPlaybookBoost