docs: rule len and cap as an i32 stability carve-out

lib/regex/whitebox_test.ww

@@ -39,7 +39,7 @@ type texp = struct {
 	start_bytesize: size,
 	matched: bool,
 	failed: bool,
-	// .len reads as i32 (check.c:1239), so the count columns match it
+	// Root CLAUDE.md rule 9 keeps .len at i32, so these counts use i32.
 	ncaps: i32,
 	nreps: i32,
 };