wcc: tagged-union foundations (never, void, flatten, exhaust)

- `never` bottom type: TY_NEVER, assignable to anything; size 0.
- Type-set normalization for N_TTAGGED in resolve_type:
  - flatten nested anonymous (A|B)|C → (A|B|C); named aliases stay
    nominal (not flattened through)
  - dedup duplicates (NAMED pointer-id; others structural)
  - drop `never` variants
  - collapse single-element set: (T|never) → T, (T|T) → T
- Match exhaustiveness: error when a variant is unhandled and no
  default arm covers it. Multi-pattern `case T1 | T2 =>` counts
  each alt.
- (T | void) optionals: bare `return;` from a tagged-union-returning
  fn emits the void variant's tag (payload undefined; void size 0).

selfhost mirrored: TY_NEVER constant + tynever in tctx + seedprim
entry; voidvariantindex helper; cgreturn bare-return handling.

selfhost/cmd/wcc/cgenutil.ww

@@ -876,6 +876,24 @@ fn istaggedtype(t: *node) bool = {
 	return false;
 };
 
+// voidvariantindex — find the 0-based index of the `void` variant in a
+// tagged-union type expr, -1 if absent. Used by cgreturn to map bare
+// `return;` in a tagged-union-returning fn to the void variant's tag.
+fn voidvariantindex(tagged: *node) i32 = {
+	if (tagged == nil) { return -1; };
+	if (tagged.kind != N_TTAGGED) { return -1; };
+	let v: *node = tagged.list;
+	let idx: i32 = 0;
+	for (v != nil) {
+		if (v.kind == N_TNAME) {
+			if (streq(v.str, "void")) { return idx; };
+		};
+		v = v.next;
+		idx += 1;
+	};
+	return -1;
+};
+
 // rhstargetname — for a returned value, what's its declared (or
 // surface-inferred) type name? `expr: T` casts dictate T directly;
 // bare strlit/intlit fall back to a primitive name.