cmd/w6c/cgen+test: skip dead TRYPROP propret on same-shape ?

Per CLAUDE.md rule 10, align cstage down to wwstage — when every
variant in a `?` propagation maps to itself, the remap loop emits
zero JMPs and the propret label is dead. Lazy-allocate it so the
label-counter ID is only consumed when at least one JMP fires.

Smoke test selfhost/test/trypromote.ww exercises same-shape
(i64|nomem)→(i64|nomem) propagation; cstage and wwstage now emit
byte-identical asm for the TRYPROP region.

selfhost/test/trypromote.ww

@@ -0,0 +1,54 @@
+// selfhost/test/trypromote.ww — smoke for `?` propagation through a
+// `!void`-shaped alias.
+//
+// Task #2 (ww-strings-redesign): proves cgen's TRYPROP tag-remap works
+// for the exact pattern lib/errors + os.alloc are about to lean on. We
+// cannot smoke this against lib/ today because there are zero lib-side
+// `?` users on a `!void` alias yet. Mirrors the shlex.syntaxerr shape
+// (lib/shlex/shlex.ww:112) for the "nomem" stub.
+//
+// Not table-driven on purpose: same-shape `?` is a single cgen emit
+// pattern, so varying the operand exercises the same asm. The two
+// match arms below cover both runtime outcomes (success unwrap, error
+// propagation); asm-level regressions of task #18 are gated by the
+// byte-identity tests (994_w6c_ww, 995_self_rebuild).
+
+package test;
+
+import fmt;
+
+type nomem = !void;
+
+fn stub(fail: i64) (i64 | nomem) = {
+	if (fail != 0i64) { let e: nomem; return e; };
+	return 42i64;
+};
+
+fn caller(fail: i64) (i64 | nomem) = {
+	let v = stub(fail)?;
+	return v + 1i64;
+};
+
+export fn main() i32 = {
+	let rc: i32 = 0;
+	match (caller(0i64)) {
+	case let n: i64 => {
+		fmt.println("ok ", n);
+		if (n != 43i64) { rc = 1; };
+	};
+	case nomem => {
+		fmt.println("unexpected nomem on ok path");
+		rc = 2;
+	};
+	};
+	match (caller(1i64)) {
+	case let n: i64 => {
+		fmt.println("unexpected ", n, " on err path");
+		rc = 3;
+	};
+	case nomem => {
+		fmt.println("nomem as expected");
+	};
+	};
+	return rc;
+};