wcc/check: #150 inferred-type module-global Sym repoint (cstage, #18)

Every inferred-type module-global -- let n = 5; ... return n, or
let g = pt{...}; g.a -- yielded <nil> downstream in cstage: the module
N_LET pass-2 stamped d->type from the initializer but never repointed the
Sym, so later references resolved the still-unstamped Sym. The wwstage
checker already repointed correctly, so this aligns cstage UP (cstage-only;
no combined.ww / check.ww change, w6c_ww unchanged).

Mirrors the #11 [_]-array repoint. The fix is shape-agnostic (keyed on the
unstamped Sym, not the use site) -- verified across scalar/field/arg/index/
str/match/nested inferred-global shapes. Commit B of #150 (Commit A 5c37648
fixed the by-value struct-arg cgen). A pure inferred ARRAY global is now
correct in cstage but trips wwstage asserttyped -- opposite-stage, filed
#125-class. byte-id 990-997 8/8. test/wcc/823 table-driven.

Makefile

@@ -251,6 +251,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_arr_zero_vs_infer \
 	  $(BIN)/test_def_str_index_reject \
 	  $(BIN)/test_struct_global_byval_arg \
+	  $(BIN)/test_inferred_global_let \
 	  $(BIN)/test_slice_str_global_zero \
 	  $(BIN)/test_slice_literal_global \
 	  $(BIN)/test_global_arr_elem_field \
@@ -665,6 +666,12 @@ $(BIN)/test_struct_global_byval_arg: test/wcc/822_struct_global_byval_arg.c $(BI
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_inferred_global_let: test/wcc/823_inferred_global_let.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_def_arr_infer_len: test/wcc/814_def_arr_infer_len.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \