String lengths and found indices are now int32 facts for the integer
range analysis: runtime strings hold at most SCR_STR_MAX_CAP UTF-8 bytes,
and a UTF-16 length never exceeds the UTF-8 byte length. Loops bounded
by a string length therefore get integer counters, and charCodeAt at the
counter counts as an integer consumer for counted-loop versioning.
A charCodeAt whose index the backend holds as an exact integer calls
@sc_str_char_code_at_int: on a proven-ASCII string one unsigned compare
against the length rejects negative and out-of-range indices, and every
other case defers to scr_str_char_code_at with the exact double.
Chris Tate