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/cmd/wcc/check.ww

@@ -714,7 +714,9 @@ fn primtypesize(nm: str) i64 = {
 	if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
 	if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64")) { return 8i64; };
 	if (streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
-	if (streq(nm, "str")) { return 16i64; }; // sizelint-ok: SSoT for ty_str primtype (#64)
+	// str IS []u8: 24B, sourced from the slice header SSoT so str and
+	// []u8 can never drift; no second hardcoded 24 (#1/Phase 3).
+	if (streq(nm, "str")) { return tyslicesize(); };
 	return -1i64;
 };