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/crossmod.ww

@@ -0,0 +1,12 @@
+// crossmod — sibling module whose probe() takes a param named like the
+// module `shadowmod` that paramshadowmod (NOT crossmod) imports. The
+// shadow rule is filtered by the BINDING's own module (src_imports
+// cur_mod filter): crossmod carries no `import shadowmod`, so this param
+// must NOT trip even though a sibling module in the same bundle imports
+// that leaf. Legit-form survivor of the abolished self-import tolerance.
+
+package crossmod;
+
+export fn probe(shadowmod: str) i32 = {
+	return shadowmod.len;
+};