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.

lib/ww/typ.ww

@@ -42,12 +42,13 @@ def TY_NAMED:    i32 = 24;
 def TY_TUPLE:    i32 = 25;
 def TY_TAGGED:   i32 = 26;
 def TY_ERR:      i32 = 27;
-def TY_UNTYPED_INT:   i32 = 28;
-def TY_UNTYPED_FLOAT: i32 = 29;
-def TY_UNTYPED_STR:   i32 = 30;
-def TY_UNTYPED_RUNE:  i32 = 31;
-def TY_UNTYPED_BOOL:  i32 = 32;
-def TY_UNTYPED_NIL:   i32 = 33;
+def TY_NEVER:    i32 = 28;
+def TY_UNTYPED_INT:   i32 = 29;
+def TY_UNTYPED_FLOAT: i32 = 30;
+def TY_UNTYPED_STR:   i32 = 31;
+def TY_UNTYPED_RUNE:  i32 = 32;
+def TY_UNTYPED_BOOL:  i32 = 33;
+def TY_UNTYPED_NIL:   i32 = 34;
 
 // ---- tinfo / tfield / tparam -----------------------------------------
 
@@ -100,6 +101,7 @@ type tctx = struct {
 	tyf64:   *tinfo,
 	tystr:   *tinfo,
 	tyerr:   *tinfo,
+	tynever: *tinfo,
 	tyuntypedint:   *tinfo,
 	tyuntypedfloat: *tinfo,
 	tyuntypedstr:   *tinfo,
@@ -144,6 +146,7 @@ export fn typesinit(c: *tctx, a: *arena) void = {
 	c.tyf64    = prim(a, TY_F64,  "f64",   8u64, 8u64);
 	c.tystr    = prim(a, TY_STR,  "str",  16u64, 8u64);
 	c.tyerr    = prim(a, TY_ERR,  "<err>", 0u64, 1u64);
+	c.tynever  = prim(a, TY_NEVER, "never", 0u64, 1u64);
 
 	c.tyuntypedint   = prim(a, TY_UNTYPED_INT,   "untyped_int",   0u64, 1u64);
 	c.tyuntypedfloat = prim(a, TY_UNTYPED_FLOAT, "untyped_float", 0u64, 1u64);