selfhost: structlookup/enumlookup mod-filter (mirror aliaslookup)

Tags structinfo/enumtype with originating module; exact-match first,
then split pkg.X and filter by smod/emod. Without this, two modules
with same-leaf-name struct/enum types collapsed to whichever entry
appeared first in the chain.

Wired into 696_modtype_leaf_collision via a wwstage run_pos using
ww_ww (negative case omitted: w6c_ww has no checkfile pass). Updated
the test's Makefile deps to include the wwstage binaries.

Audited the rest of the lookup family — fnretlookup, fnparamslookup,
deflookup don't need the same treatment: the parser emits N_DOT.str
(call/field name) as the leaf only, and fnparamslookup is only
invoked with N_IDENT.str. Dotted module-qualified function calls go
through the module-mangling path instead.

Makefile

@@ -356,6 +356,7 @@ $(BIN)/test_match_bind_struct: test/wcc/695_match_bind_struct.c $(BIN)/ww \
 
 $(BIN)/test_modtype_leaf_collision: test/wcc/696_modtype_leaf_collision.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<