selfhost: cgen for enum (member fold + as pass-through)

w6c_ww now compiles enum end-to-end and emits byte-identical
asm to the C w6c on the new 994 corpus case (`type mode = enum u8
{ R, W, RW = R | W }; main() { return (mode.RW): i32 }`). Mechanism
mirrors the C side:

- collectenums walks every `type X = enum {...}` at file scope and
  pre-resolves each member's u64 value (auto-increment from prior,
  sibling-ref folding for `RDWR = READ | WRITE`).
- cgdot recognises `EnumName.MEMBER` before the local lookup and
  emits MOVQ $value, AX directly.
- cgtypeassert short-circuits when either side is enum: cgexpr on
  the LHS lands the value in AX with the right integer width; no
  tag/unwrap.

main.combined.ww (wwdump/ + w6c/) regenerated by ww build.

test/wcc/994_w6c_ww.c

@@ -143,6 +143,12 @@ main(void)
 		  "  return n * fact(n - 1);\n"
 		  "};\n"
 		  "fn main() i32 = { return fact(5); };" },
+		{ "enum",
+		  "type mode = enum u8 { R = 1, W = 2, RW = R | W };\n"
+		  "fn main() i32 = {\n"
+		  "  let m: mode = mode.RW;\n"
+		  "  return m as i32;\n"
+		  "};" },
 		{ NULL, NULL },
 	};