selfhost+test: route callee_variadic_param N_DOT via fnparamslookupmod (#16)

Latent silent miscompile surfaced by worker-strcontains3 attempting
strings.contains tagged-variadic graduation: wwstage cgcall's
callee_variadic_param helper (cgenutil.ww:60-70) consumed the N_DOT
callee's leaf via callee.str but routed bare fnparamslookup — bypassed
the module hint at callee.lhs.str. When two modules export same-leaf
fns with differing variadic shapes (e.g. strings.contains(str|rune)...
+ bytes.contains scalar (u8|[]u8)), the bare walk returned the wrong
fn's params for arg-prep while the CALL targeted the correct
module-qualified symbol — ABI mismatch.

Direct sister of #34 (049ebc1) which graduated fnret's N_DOT arm
through fnretlookupmod. #4d's commit body (862715d) explicitly
deferred callee_variadic_param's *mod re-routing pending "future
stdlib port introducing a tagged-vs-scalar or variadic-vs-non-variadic
same-leaf N_DOT collision shape." This is that surfacing.

cgenutil.ww: split callee_variadic_param on callee.kind. N_IDENT stays
on bare fnparamslookup (same-module-first post-#4d). N_DOT routes
through fnparamslookupmod(c, callee.str, callee.lhs.str), pattern-
identical to cgcall's N_DOT branch at cgenexpr.ww:2922-2935.

Cstage cmd/w6c/cgen.c:4279-4302 reads callee params via typed AST
(n->lhs->type + cu->params) — module-aware natively, no sister
change needed (mirrors #4d/#28/#31/#34 cstage no-sister notes).

752_modparam_callee: table-driven 3 rows x 2 stages = 6 fixtures.
cross_module_same_leaf_variadic_vs_scalar (the wedge),
same_module_same_leaf (no-regress), bare_leaf_no_collision (control).

#17 filed for the wider convenience-wrapper audit (enumerate all
wwstage cgen* helpers that take *node and do bare-leaf lookups; sweep
for N_DOT-arm omissions). This commit is narrow to callee_variadic_param.

make test 126/126; ww2==ww3==ww4 byte-id holds via 995_self_rebuild.

Makefile

@@ -286,6 +286,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_sumtype_forward \
 	  $(BIN)/test_mklabel_modscoped \
 	  $(BIN)/test_vararg_seq_percall \
+	  $(BIN)/test_modparam_callee \
 	  $(BIN)/test_param_shadow_mod \
 	  $(BIN)/test_localoff_scope \
 	  $(BIN)/test_cast_enum_movl \
@@ -727,6 +728,12 @@ $(BIN)/test_vararg_seq_percall: test/wcc/751_vararg_seq_percall.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_modparam_callee: test/wcc/752_modparam_callee.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 $@ $<
+
 $(BIN)/test_match_4arm_cross_module_run: test/wcc/929_match_4arm_cross_module_run.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \