feat: CheckSnapshots y UpdateSnapshot en fn_operations library

Añade UpdateTypeSnapshot al store, CheckSnapshots para comparar snapshots
locales vs registry (up_to_date/outdated/missing), y UpdateSnapshot para
re-snapshot con retorno de old/new para diffing.

fn_operations/store.go

@@ -58,6 +58,20 @@ func (db *DB) InsertTypeSnapshot(ts *TypeSnapshot) error {
 	return err
 }
 
+// UpdateTypeSnapshot replaces an existing snapshot with a new version.
+func (db *DB) UpdateTypeSnapshot(ts *TypeSnapshot) error {
+	if ts.SnappedAt.IsZero() {
+		ts.SnappedAt = time.Now().UTC()
+	}
+	_, err := db.conn.Exec(`
+		UPDATE types_snapshot SET version=?, lang=?, algebraic=?, definition=?, description=?, snapped_at=?
+		WHERE id=?`,
+		ts.Version, ts.Lang, ts.Algebraic, ts.Definition, ts.Description,
+		ts.SnappedAt.Format(time.RFC3339), ts.ID,
+	)
+	return err
+}
+
 // GetTypeSnapshot returns a type snapshot by ID.
 func (db *DB) GetTypeSnapshot(id string) (*TypeSnapshot, error) {
 	row := db.conn.QueryRow("SELECT id, version, lang, algebraic, definition, description, snapped_at FROM types_snapshot WHERE id = ?", id)