Migrate NPC reconciliation to shared engine

internal/framework/semanticreconcile/preparation.go

@@ -213,7 +213,7 @@ func Prepare(document *source.SourceDocument, candidates []Candidate, limits Lim
 	}
 	windows, err := buildContextWindows(document.Units, coalesceIntervals(intervals), cited)
 	if err != nil {
-		return Preparation{}, fmt.Errorf("prepare semantic reconciliation: build transcript material: %w", err)
+		return Preparation{}, fmt.Errorf("prepare semantic reconciliation: build transcript material: invalid source metadata")
 	}
 	transcriptContent, err := json.Marshal(transcriptInput{Windows: windows})
 	if err != nil {

internal/framework/semanticreconcile/preparation_test.go

@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"math"
 	"reflect"
 	"strings"
 	"testing"
@@ -148,6 +149,27 @@ func TestPrepareBuildsContiguousCandidatesAndOwnedSourceContext(t *testing.T) {
 	}
 }
 
+func TestPrepareRedactsInvalidSourceMetadata(t *testing.T) {
+	const sensitiveKey = "sensitive-metadata-key"
+	document := &source.SourceDocument{ID: "private-source", Units: []source.SourceUnit{
+		{ID: 1, Text: "private transcript", Metadata: map[string]any{sensitiveKey: math.NaN()}},
+		{ID: 2, Text: "other private transcript"},
+	}}
+	candidates := []Candidate{
+		{Label: "Private One", SourceRefs: []source.SourceRef{{SourceID: document.ID, StartUnitID: 1, EndUnitID: 1}}},
+		{Label: "Private Two", SourceRefs: []source.SourceRef{{SourceID: document.ID, StartUnitID: 2, EndUnitID: 2}}},
+	}
+	_, err := Prepare(document, candidates, DefaultLimits())
+	if err == nil || !strings.Contains(err.Error(), "invalid source metadata") {
+		t.Fatalf("Prepare() error = %v, want redacted metadata failure", err)
+	}
+	for _, forbidden := range []string{sensitiveKey, document.ID, document.Units[0].Text, candidates[0].Label, "non-finite", "float64"} {
+		if strings.Contains(err.Error(), forbidden) {
+			t.Fatalf("Prepare() error leaked %q: %v", forbidden, err)
+		}
+	}
+}
+
 func TestPrepareFiltersUnsafeCandidatesAndCoalescesAdjacentWindows(t *testing.T) {
 	document := &source.SourceDocument{ID: "session", Units: []source.SourceUnit{
 		{ID: 9}, {ID: 3}, {ID: 8}, {ID: 1}, {ID: 7},