wcc/check: reject self-import, both stages (#16 ENFORCE-checker)

check-(c): a package importing itself (any spelling) is a hard error,
mirroring Go. Predicate is leaf==owner at the N_USE/installdecl seam —
sound only after the PREP commits (dotted-test renames, package-less
boundary directive). Identical wording both stages; diagnostics-only,
byte-id-neutral. 948 pins the reject in both compilers; 708's
pos_selfimp (which pinned the abolished self-import skip) converts to
neg_selfimp + new pos_crossmod preserving the param-shadow tolerance
the case existed for. Checks (a) unused and (b)/(d) name-membership
stay deferred to the multi-package arc: imports are filename-keyed
pulls, so those need import->file provenance this compiler lacks.

test/wcc/data/paramshadowmod/pos_crossmod.ww

@@ -0,0 +1,17 @@
+// pos_crossmod — cur_mod-filtering positive. Replaces the abolished
+// pos_selfimp (self-import is hard-rejected post-#16). This module
+// (paramshadowmod) imports `shadowmod`, and the bundle also pulls module
+// `crossmod`, whose probe() has a param named `shadowmod`. crossmod does
+// NOT import shadowmod, so the shadow rule — filtered by the binding's
+// own module — must NOT trip crossmod's param, even though a sibling
+// module in the same bundle imports that leaf. Build + run; exit = 2.
+
+package paramshadowmod;
+
+import shadowmod;
+import crossmod;
+
+export fn main() i32 = {
+	let _ = shadowmod.say();
+	return crossmod.probe("hi");
+};