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