w6c+selfhost: codegen for top-level mutable let

Third step toward writable globals. The C cgen and its selfhost
mirror now:

  - emit DATAW <name>(SB),"<8 LE bytes>" for every top-level `let`
    whose type lands in the scalar set (i8..i64/u8..u64/bool/rune/
    int/uint/uintptr/ptr; floats and multi-word types deferred);
  - drop the "no writable .data" silent-drop guard at the N_IDENT
    store path, replacing it with a RIP-relative MOVQ for `=` and
    a load→combine→store sequence for the compound ops; and
  - route `&name` through LEAQ name(SB) instead of dropping it.

Type aliases resolve via aliaslookup so `type counter = i32; let c:
counter = 0;` still emits a DATAW slot. Non-literal initialisers
silently skip, which surfaces as a clean undefined-symbol error if
the binding is ever referenced.

The selfhost mirror lands in the same commit because test 990
diffs the C cgen against wwdump_ww -c on err.ww (which has
top-level `let nerrors: i32 = 0; ... nerrors += 1;`). Any drift
between the two cgens makes 990 fail. Bootstrap stays at a fixed
point: ww2 == ww3 == ww4 byte-identical.

Makefile

@@ -214,7 +214,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_data_link \
 	  $(BIN)/test_arch \
 	  $(BIN)/test_e2e $(BIN)/test_ffi $(BIN)/test_dyn $(BIN)/test_stdlib \
-	  $(BIN)/test_at_test \
+	  $(BIN)/test_at_test $(BIN)/test_let_global \
 	  $(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
 	  $(BIN)/test_w6c_ww $(BIN)/test_ww_ww $(BIN)/test_self_rebuild \
 	  $(BIN)/test_dyn_ww $(BIN)/test_selfcheck $(BIN)/test_at_test_ww
@@ -268,6 +268,10 @@ $(BIN)/test_at_test: test/wcc/910_at_test.c $(BIN)/ww $(BIN)/w6c $(BIN)/w6a \
 		$(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_let_global: test/wcc/630_let_global.c $(BIN)/ww $(BIN)/w6c \
+		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_selfhost: test/wcc/990_selfhost.c $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww $(BIN)/wwdump $(BIN)/wwdump_ww $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<