test: extend 708 with mlet/forrange/mcase shadow rows (#28)

708 pinned param + clet + self-import-skip after #19 landed; the
other 4 sites where check_module_shadow is wired (N_MLET, N_FORRANGE
single, N_FORRANGE tuple, N_MCASE) had no dedicated negative-compile
row. Worker-19's note that all 6 sites use identical call shape is
true today but unpinned by tests; a future asymmetric edit could
silently disable enforcement on one site.

Adds neg_mlet (let-tuple binder shadows), neg_forrange_single
(`for shadowmod of ...`), neg_forrange_tuple (`for (shadowmod, x)
of pairs`), neg_mcase (`case let shadowmod:`). Each errors at the
binder decl line with the canonical
"<kind> '<name>' shadows imported module '<name>'" message.

708's main now runs 8/8 (param, let, mlet, forrange_single,
forrange_tuple, mcase, pos_rename, pos_selfimp).

test/wcc/data/paramshadowmod/neg_mcase.ww

@@ -0,0 +1,23 @@
+// neg_mcase — `match (r) { case let shadowmod: i64 => ... }` where
+// the per-arm binder shadows the imported module. N_MCASE wires
+// check_module_shadow before scope_define on cs->str, so the rule
+// fires at the case line.
+
+use shadowmod;
+
+fn parse(n: i64) (i64 | i32) = {
+	if (n < 0i64) {
+		return 1i32;
+	};
+	return n;
+};
+
+export fn main() i32 = {
+	let r: (i64 | i32) = parse(42i64);
+	let out: i64 = 0i64;
+	match (r) {
+	case let shadowmod: i64 => out = shadowmod;
+	case let e: i32 => return e;
+	};
+	return out: i32;
+};