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

lib/CLAUDE.md

@@ -12,7 +12,7 @@ Signatures mirror Hare too, modulo:
 
 - Tagged-union returns are spelled with the ww `!` error tag where
   Hare uses `!void` / `!T`, and indices use the underlying length
-  type (`i32` today, since `str.len: i32`). Example:
+  type (`i32` under root CLAUDE.md rule 9). Example:
   `strconv.stoi64(s: str, b: strconv.base) (i64 | invalid | overflow)`
   — same shape as Hare's. The base parameter is the named enum
   `strconv.base` (Hare uses `enum uint`; we pick `enum i32` to

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,
 };