selfhost+cstage+test: graduate deflookup/deflookuprhs same-module-first (#4c)

Class A silent miscompile, latent until two modules export the same
str-typed def leaf name and the .ptr/.len field-fold path consumes
the wrong-module strlit address/length. Wwstage's deflookuprhs
(selfhost/cmd/wcc/cgen.ww) walked c.defs head-first by dname; cgdot's
.ptr/.len field-fold handed it the bare leaf from N_IDENT.str,
silently inlining the wrong-module strlit. Cstage carries the same
shape at cmd/w6c/cgen.c (Sdef walk #3 N_DOT field-fold): Sdef keyed
by name only, head-pick on every cross-module collision. No in-tree
corpus declares two same-leaf str defs, so 995_self_rebuild stayed
green (same surfacing pattern as #4a enumlookup post-strings and
#4b structlookup).

Sixth leaf of the trio leaf-name lookup graduation (after #27
aliaslookup, #28 fnparams, #31 fnret, #4a enum, #4b struct). Same
bundle precedent as #4a (which bundled wwstage enumlookup +
enumlookupmod + cstage scope_lookup_prefer sister fix under one
structural concern): four sister changes ship together.

  - defent +dmod field; collectdefs captures d.module.
  - wwstage deflookup two-pass walk — cosmetic (bool return is
    invariant under head-pick vs same-module-first), kept for
    structural symmetry with deflookuprhs.
  - wwstage deflookuprhs two-pass walk — load-bearing for the
    .ptr/.len field fold.
  - cstage Sdef +mod field; sdef_collect captures d->module raw
    (matches cgfn's raw cur_mod convention); new sdef_mod_match
    helper handles NULL-safe strcmp; cstage Sdef walk #3 N_DOT
    field-fold graduation (sister of wwstage deflookuprhs).

Two additional cstage Sdef walks (N_IDENT bare load + N_DOT mod-
qualified fallback) are DEFERRED. Both consume wwstage's
cgenexpr.ww:553 path which is independently broken (str-def bare/
qualified reference emits MOVQ symname(SB) where strlit-inline is
required); sentinel rows for those walks fail cs-vs-ws byte-id
regardless of the cstage prefer-pass behavior. Per rule 7 the
prefer-pass cannot ship without sentinels. Filed: task #11 (cstage
walk #2 also needs n->lhs->str as hint source rather than cur_mod,
matching #4a/#28/#31's *mod variant pattern) + task #12 (wwstage
str-def symbol-load fix that unblocks both deferrals).

735_def_modshadow pins the fix with 1 row: bare-leaf .len of MSG
in module alpha must fold against alpha's own def MSG (strlit
length 41) even with beta's same-leaf 27-char def MSG at the head
of c.defs / sdefs. Asserts the matching immediate inside the right
TEXT sym + bad_imm anti-check on both stages plus byte-id between
stages.

Makefile

@@ -264,6 +264,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_match_4arm_cross_module_run \
 	  $(BIN)/test_enum_modshadow \
 	  $(BIN)/test_struct_modshadow \
+	  $(BIN)/test_def_modshadow \
 	  $(BIN)/test_param_shadow_mod \
 	  $(BIN)/test_localoff_scope \
 	  $(BIN)/test_cast_enum_movl \
@@ -618,6 +619,10 @@ $(BIN)/test_struct_modshadow: test/wcc/734_struct_modshadow.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_def_modshadow: test/wcc/735_def_modshadow.c \
+		$(BIN)/w6c $(BIN)/w6c_ww | $(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 \