feat(wing): backtest metric (coverage/precision at a threshold) and shared TopWing (#205 S1)

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

ingestion/internal/wing/backtest.go

@@ -0,0 +1,43 @@
+package wing
+
+// Sample is one backtest observation: the human-decided wing of an existing
+// page, and what the two independent opinions said about it with the wing
+// label removed.
+type Sample struct {
+	Truth      string
+	Suggested  string // classifier's wing, "" if it abstained or failed
+	Confidence float64
+	ScorerTop  string // scorer's single best wing, "" if no clear opinion
+}
+
+// Result is the policy outcome at one threshold.
+type Result struct {
+	N         int
+	Auto      int     // placed without a human
+	Correct   int     // of Auto, matching Truth
+	Coverage  float64 // Auto / N
+	Precision float64 // Correct / Auto; 0 when Auto == 0
+}
+
+// Evaluate applies the Resolver's classified-path rule (confident, and the
+// scorer agrees) to recorded samples. Eligibility is not modelled: callers
+// backtest only on wings they would allow.
+func Evaluate(samples []Sample, threshold float64) Result {
+	r := Result{N: len(samples)}
+	for _, s := range samples {
+		if s.Suggested == "" || s.Confidence < threshold || s.ScorerTop != s.Suggested {
+			continue
+		}
+		r.Auto++
+		if s.Suggested == s.Truth {
+			r.Correct++
+		}
+	}
+	if r.N > 0 {
+		r.Coverage = float64(r.Auto) / float64(r.N)
+	}
+	if r.Auto > 0 {
+		r.Precision = float64(r.Correct) / float64(r.Auto)
+	}
+	return r
+}