Replace str_char_at (returns strdup String) with str_char_code (returns Int)
in the main lex() while loop and scan_digits/scan_ident helpers.
For a 400KB combined source, str_char_at was allocating ~400K x 16B = 6.4MB
of transient 2-byte strings for the ch variable alone. str_char_code returns
an integer directly — zero allocation.
Add Int-based helpers: is_digit_code, is_alpha_code, is_ws_code,
is_alnum_or_underscore_code. Rewrite lex() operator dispatch using char
code constants (e.g. '/'=47, '"'=34, '='=61).
Result on main.el: 17.1MB -> 15.4MB peak RSS (-10%).
Self-hosting: PASS.
Will Anderson