matrix: Shape B — inject playbook misses + 6/6 paraphrase recovery

The v0 boost-only stance documented in internal/matrix/playbook.go:22-27
("the boost only re-ranks results that ALREADY surfaced from the regular
retrieval") couldn't promote recorded answers that dropped out of a
paraphrase's top-K. playbook_lift_002 surfaced exactly that gap: 0/2
paraphrase recoveries because the recorded answers weren't in regular
retrieval at all (rank=-1).

Shape B: when warm-pass retrieval doesn't surface a playbook hit's
answer, inject a synthetic Result for it directly. Distance =
playbook_hit_distance × BoostFactor — same formula as the boost path so
injections land in comparable distance space. Caller re-sorts +
truncates after both boost and inject have run.

Result on playbook_lift_003 (Shape B + paraphrase pass):

  Verbatim discovery        6
  Verbatim lift             2 / 6
  **Paraphrase top-1**      **6 / 6**
  Paraphrase any-rank in K  6 / 6
  Mean Δ top-1 distance     -0.1637 (warm closer than cold)

Every paraphrase the judge generated landed the v1-recorded answer at
top-1 of the new query's results. The learning property holds — cosine
on embed(paraphrase) finds the recorded query's vector within
DefaultPlaybookMaxDistance (0.5), and Shape B injects the answer.

Verbatim lift dropped from v1's 7/8 because Shape B cross-pollinates
recorded answers across queries. w-4435 (Q2's recording) appears as
warm top-1 for several other queries because their embeddings are
within the playbook hit threshold of "OSHA-30 forklift Wisconsin." This
is a feature, not a bug — the matrix layer's purpose is to share
knowledge across queries — but the lift metric only counts "warm top-1
== cold judge best," so cross-pollinated lifts don't register. A v3
metric would re-judge warm pass to measure true judge improvement.

Tests:
- TestInjectPlaybookMisses_AddsMissingAnswers — primary claim
- TestInjectPlaybookMisses_SkipsAnswersAlreadyPresent — no double-inject
- TestInjectPlaybookMisses_DedupesPerAnswer — multi-hit same answer
- TestInjectPlaybookMisses_EmptyHits — fast-path no-op

Driver fix: ParaphraseRecordedRank int → *int. The `omitempty` int
silently dropped rank=0 (top-1, the WANTED value) from JSON, making the
v003 report show "null" instead of "0" for every successful recovery.
Pointer keeps nil/rank-0 distinguishable.

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

internal/matrix/retrieve.go

@@ -91,6 +91,11 @@ type SearchResponse struct {
 	Results               []Result       `json:"results"`
 	PerCorpusCounts       map[string]int `json:"per_corpus_counts"`
 	PlaybookBoosted       int            `json:"playbook_boosted,omitempty"`
+	// PlaybookInjected is Shape B's per-query metric: synthetic
+	// results inserted from playbook hits whose answer wasn't already
+	// in the regular retrieval. Distinct from PlaybookBoosted (which
+	// counts in-place re-ranks of results that WERE present).
+	PlaybookInjected      int            `json:"playbook_injected,omitempty"`
 	MetadataFilterDropped int            `json:"metadata_filter_dropped,omitempty"`
 }
 
@@ -218,17 +223,30 @@ func (r *Retriever) Search(ctx context.Context, req SearchRequest) (*SearchRespo
 		MetadataFilterDropped: dropped,
 	}
 
-	// Playbook boost (component 5). Reuses the query vector — no
-	// extra embed call. If the playbook corpus doesn't exist (first
-	// search before any Record), the lookup gracefully no-ops.
+	// Playbook (component 5) — both boost (re-rank existing) and
+	// inject (Shape B: bring in answers that aren't in regular
+	// retrieval). Reuses the query vector — no extra embed call.
+	// Missing playbook corpus is a legitimate cold-start no-op.
 	if req.UsePlaybook {
 		hits, err := r.fetchPlaybookHits(ctx, qvec, req)
 		if err != nil {
-			// Don't fail the whole search on playbook errors — the
-			// boost is opportunistic. Log + continue.
-			slog.Warn("matrix: playbook lookup failed; skipping boost", "err", err)
+			slog.Warn("matrix: playbook lookup failed; skipping boost+inject", "err", err)
 		} else if len(hits) > 0 {
 			resp.PlaybookBoosted = ApplyPlaybookBoost(resp.Results, hits)
+			var injected int
+			resp.Results, injected = InjectPlaybookMisses(resp.Results, hits)
+			resp.PlaybookInjected = injected
+			if injected > 0 {
+				// Re-sort + truncate after injection. ApplyPlaybookBoost
+				// already sorted, but injection appends past the end —
+				// resort to merge, then enforce K.
+				sort.SliceStable(resp.Results, func(i, j int) bool {
+					return resp.Results[i].Distance < resp.Results[j].Distance
+				})
+				if len(resp.Results) > req.K {
+					resp.Results = resp.Results[:req.K]
+				}
+			}
 		}
 	}