Parses byte-identical to the C frontend on enum sources (verified
via `diff` of wwdump vs wwdump_ww -a on an enum-using fixture).
The selfhost side reserves the slot in the AST and TY_* enums so
later check.ww and cgen mirror work doesn't shift numeric IDs.
Codegen-side enum support (member-value folding in cgdot, enum↔int
pass-through in cgtypeassert) is deferred — current selfhost sources
don't use enum, so 990_selfhost / 995_self_rebuild stay green.
main.combined.ww in wwdump/ and w6c/ regenerated by `ww build` as
a side effect of `make wwstage`.
Hojun-Cho
5bf30f209c