Wrapped compile() body in el_arena_push/pop so the arena is active
before lex() and scan_fn_sigs(). Previously both ran with
_tl_arena_active=0, leaking all token and signature strings permanently.
Also prevents inner pop(mark=0) calls from deactivating the arena
between per-function scopes. Verified: self-host PASS, RSS stable.
Combines two orthogonal optimizations:
1. Flat token list (from beta): lex() returns [Any] with alternating kind/value
pairs instead of [Map], eliminating one ElMap per token (~3 mallocs each).
Parser updated: tok_kind(t,i) = t[2*i], tok_value(t,i) = t[2*i+1].
2. Char code dispatch (from alpha): lex() hot loop uses str_char_code -> Int
instead of str_char_at -> strdup String for all character classification.
Eliminates ~400K x 16B = 6.4MB of temporary string allocations.
scan_digits and scan_ident also updated to use str_char_code.
Result on main.el: 17.1MB -> 14.4MB peak RSS (-16%).
Self-hosting: PASS.
Will Anderson