Move chunks into the canonical source model

internal/core/source/digest.go

@@ -33,3 +33,33 @@ func DigestDocument(doc *SourceDocument) (string, error) {
 	sum := sha256.Sum256(encoded)
 	return "sha256:" + hex.EncodeToString(sum[:]), nil
 }
+
+// DigestChunk returns a deterministic digest of a chunk, including its source
+// provenance, content, units, and metadata.
+func DigestChunk(chunk Chunk) (string, error) {
+	payload := struct {
+		ID        string         `json:"id"`
+		SourceID  string         `json:"source_id"`
+		Index     int            `json:"index"`
+		Ref       SourceRef      `json:"ref"`
+		Content   []byte         `json:"content"`
+		MediaType string         `json:"media_type"`
+		Units     []SourceUnit   `json:"units"`
+		Metadata  map[string]any `json:"metadata,omitempty"`
+	}{
+		ID:        chunk.ID,
+		SourceID:  chunk.SourceID,
+		Index:     chunk.Index,
+		Ref:       chunk.Ref,
+		Content:   chunk.Content,
+		MediaType: chunk.MediaType,
+		Units:     chunk.Units,
+		Metadata:  chunk.Metadata,
+	}
+	encoded, err := json.Marshal(payload)
+	if err != nil {
+		return "", fmt.Errorf("encode source chunk for digest: %w", err)
+	}
+	sum := sha256.Sum256(encoded)
+	return "sha256:" + hex.EncodeToString(sum[:]), nil
+}