compiler: port Parser.res -> Parser.affine (recursive-descent parser)

Fourth and largest module (1150 lines). Complete error-tolerant recursive-descent
parser, faithful grammar and precedence. Functional state-threading: ParserState
is immutable; producers return (Option<X>, ParserState) and callers thread via
`let (x, st) = f(st)`; ReScript `ref` accumulators became `let mut` locals with
(acc, state) while-loops.

Full coverage: the expression ladder (ternary -> logical-or/and -> equality ->
comparison -> term -> factor -> unary -> postfix -> primary), array/lambda/grouped
primaries, call/index/member postfix chains; type expressions incl. Echo<A,B> /
EchoR<A,B> and the >>-split close-angle; the full statement set incl. gutter-block
error recovery; function/struct/main declarations; and the `parse` driver returning
(Program, [Diagnostic]). Verified green with `affinescript check` and the full
harness (Types/Lexer/Cst/Parser all ok).

Documented faithful deviations: the >>-split token rewrite reproduced via array
rebuild (tokens are immutable); the pre-existing node-`start` quirk reproduced
exactly rather than corrected; minor error-recovery loop hardening to guarantee
progress on a stray token.

AffineScript checker bug found + worked around: an if-statement immediately
followed by a tuple literal in tail position fails with E0104 "Expected a function,
got Unit" (the if's () is applied to the tuple). Used explicit `return (...)`.
Third affinescript bug this migration (after the module-resolver struct-field gap
and the global `len` builtin-shadow); all worth upstreaming.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0195yA45jSSP7YDPwJSpw4bM

Author: hyperpolymath