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/cgenstmt.ww

@@ -120,9 +120,21 @@ fn cgreturn(c: *cgen, n: *node) void = {
 		};
 		cgexpr(c, rhs);
 	} else {
-		// Bare `return;` in a void fn — zero AX so the caller
-		// sees a deterministic value (matches C cgen, which
-		// always falls through to `cgexpr_int(c, 0)`).
+		// Bare `return;` from a tagged-union-returning fn is
+		// the void variant: emit its tag. Payload is undefined
+		// (void has size 0). Otherwise zero AX for determinism.
+		if (istaggedtype(c.fnret)) {
+			let idx: i32 = voidvariantindex(c.fnret);
+			if (idx < 0) { idx = 0; };
+			emitline("\tMOVQ\t$");
+			emitint(idx: i64);
+			emitline(", AX\n");
+			emitline("\tMOVQ\tBP, SP\n");
+			emitline("\tPOPQ\tBP\n");
+			emitline("\tRET\n");
+			c.lastwasreturn = 1;
+			return;
+		};
 		emitline("\tMOVQ\t$0, AX\n");
 	};
 	// SysV: 16-byte aggregates (str, 2-tuple) return in (AX, DX).