feat(wing): score a wing by its best page, not summed hits; add wingbacktest command (#205 S1)

A first backtest showed BM25 summed over a wing's top hits favours big wings
(0/4 agreement). Best-page scoring is volume-robust; pinned by a test that
failed before the change.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ingestion/internal/wing/bm25scorer.go

@@ -11,7 +11,6 @@ import (
 const (
 	maxQueryTerms = 12
 	minTermLen    = 5
-	hitsPerWing   = 3
 )
 
 // BM25Scorer scores a note against each wing by running the brain's own BM25
@@ -38,24 +37,18 @@ func (b *BM25Scorer) Score(text string) map[string]float64 {
 		return out
 	}
 	for _, w := range b.wings {
-		hits, err := search.Query(b.brainDir, search.QueryOptions{Query: query, Wing: w, Limit: hitsPerWing + 1})
+		hits, err := search.Query(b.brainDir, search.QueryOptions{Query: query, Wing: w, Limit: 2})
 		if err != nil {
 			continue
 		}
-		var sum float64
-		n := 0
 		for _, h := range hits {
 			if b.exclude != "" && h.Path == b.exclude {
 				continue
 			}
-			if n == hitsPerWing {
-				break
-			}
-			sum += float64(h.Score)
-			n++
-		}
-		if sum > 0 {
-			out[w] = sum
+			// Hits come best-first: a wing is scored by its single best
+			// page, so a big wing cannot win on volume alone.
+			out[w] = float64(h.Score)
+			break
 		}
 	}
 	return out

ingestion/internal/wing/bm25scorer_test.go

@@ -43,3 +43,19 @@ func TestBM25Scorer_NoUsableTermsScoresNothing(t *testing.T) {
 	s := wing.NewBM25Scorer(scorerBrain(t), []string{"infra", "finance"}, "")
 	assert.Empty(t, s.Score("a an to of"))
 }
+
+// A big wing must not win just by having more pages that brush the query: a
+// wing is scored by its single best page, not by the sum of its top hits.
+func TestBM25Scorer_ABigWingDoesNotWinOnVolume(t *testing.T) {
+	dir := t.TempDir()
+	writePage(t, dir, "wiki/small/facts/target.md", "---\ntitle: Target\n---\nkubernetes cluster scheduling pods deployment nodes\n")
+	for i := 0; i < 8; i++ {
+		writePage(t, dir, filepath.Join("wiki/big/facts", "p"+string(rune('a'+i))+".md"),
+			"---\ntitle: Filler\n---\nkubernetes cluster scheduling mentioned in passing among bookkeeping ledger entries\n")
+	}
+	s := wing.NewBM25Scorer(dir, []string{"small", "big"}, "")
+	got := s.Score("kubernetes cluster scheduling pods deployment nodes")
+	top, ok := wing.TopWing(got)
+	assert.True(t, ok)
+	assert.Equal(t, "small", top)
+}