wcc: str -> 24B {ptr,len,cap}, 3-reg ABI -- parity with []u8 (both stages)

A ww `str` becomes a 24-byte {ptr,len,cap} value, identical in layout to
[]u8 -- the enabling prerequisite for the Phase 2 `str == []u8` collapse.

Both stages, atomically:
- ty_str 16->24B; str value flows 3-reg AX/BX/CX (was 2-reg); str literals
  emit cap (=len).
- str in a tagged union grows to a 32B slot, using the AX/DX/CX/R8 4th-word
  path already used by 32B slice-variant unions -- str-variant is now
  structurally identical.
- tuple (scalar,str) return: 4-reg AX/DX/CX/R8 + 32B receive, extending the
  existing type-keyed return (no sret).
- str == []u8 for index and .ptr/.len/.cap, kind-gated where size-based
  dispatch collided at 24B; cstage and wwstage mirror exactly.
- table-driven runtime coverage: test/wcc/928_str_abi_run.c.

Cannot be split (rule 10/11): a 24B str and a 16B str cannot coexist across
the two compiler stages without breaking byte-identity, so the size change
and every dependent ABI/codegen site land in one atomic commit, both stages.

Known follow-ups (zero corpus impact, tracked): str-literal global .cap
static-init; >16B struct by-value (pre-existing); tagged-union
match-scrutinee stage divergence (pre-existing).

selfhost/test/smoke.combined.ww

@@ -3365,7 +3365,7 @@ export fn main() i32 = {
 	// (#42). Each call folds to an N_INTLIT at check time; cgen
 	// materialises the literal as a plain `MOVQ $N, AX`. Mirrors
 	// cstage cmd/wcc/check.c:907-960 byte-for-byte on this corpus.
-	if (size(str) != 16) { return 23; };
+	if (size(str) != 24) { return 23; };	// str IS []u8: {ptr,len,cap} 24B (#1/Phase 3)
 	if (size(i64) != 8)  { return 24; };
 	if (size(i32) != 4)  { return 25; };
 	if (align(i64) != 8) { return 26; };