strings: preserve UTF-8 and boundary invariants

lib/CLAUDE.md

@@ -40,9 +40,9 @@ Signatures mirror Hare too, modulo:
 - `(T | U)` sum-typed parameters dispatch via `match` inside the
   callee. `strings.byteindex(haystack: str, needle: (str | rune))`,
   `bytes.index(s: []u8, needle: (u8 | []u8))`, and `rbyteindex`/
-  `rindex` follow Hare's shape directly. The rune-indexed
-  `strings.index` (rune-wise position) isn't shipped yet — we don't
-  have UTF-8 rune iteration in the language stack.
+  `rindex` follow Hare's shape directly. `strings.index`/`rindex` expose
+  rune positions; `byteindex`/`rbyteindex` expose byte offsets. UTF-8
+  iterators and both indexing axes are shipped and tested.
 
 Don't ship a richer surface than Hare has. A documented subset is
 fine; an extension, rename, or convenience-wrapper is not — callers