w6c+w6c_ww: global-base arm for indexed struct-element field read (fix #21)

The `arr[i].field` N_DOT read branch in both stages was gated on a LOCAL
base lookup (cstage `localfind != 0`, wwstage `localfindnode != nil`). A
module-GLOBAL base (`let g: [2]pt = [...]`) missed it:

  - cstage fell to a generic index-load that drops f->offset — it read
    element[i] at offset 0, so `g[i].b` returned a's value (g[0].b -> 1,
    g[1].b -> 3 instead of 2, 4).
  - wwstage fell to the module-qualified SB fallback — garbage, no main.g
    load at all.

Silent, byte-id-divergent. This is the READ twin of #11 (the global
`g[i] = v` write fix) and the #15 sibling. Local `[N]struct` bases read
correctly (tests 680/681 cover only those), which is why it was never
caught.

Fix (both stages, converged byte-identical): resolve the global the same
way the N_INDEX arm does — cstage `let_islet || def_isarraydef`, wwstage
`letvartnode || defvartnode` — and dispatch the base load by shape: array
-> LEAQ name(SB) (the symbol IS the storage), slice/ptr -> MOVQ name(SB)
(the symbol's first word IS the .ptr). The field then loads at f->offset
exactly as the local arm does. esz (element stride) and f->offset both
come from the type table (rule 13). Mirrors #11's write-side global-base
resolution. combined.ww embeds (w6c + wwdump) regenerate.

688_global_arr_elem_field: global `[2]pt` reads of .a/.b on both elements
(the .b reads are the bug), a non-8-aligned `[2]rec {tag:u8,x:i32,y:i64}`
to stress f->offset + a u8 sub-word leaf, and a slice-base read
(`let g: []rec = arr;`) that exercises the MOVQ-deref .ptr arm. Runtime
(cstage build+run) + cstage==wwstage byte-id per row. The slice row is
byte-id ONLY: its read asm is correct and identical on both stages, but a
slice-of-struct module global does not data-emit a symbol yet (a separate,
pre-existing data-emission gap, sibling of #10/#20), so it cannot link/run.

Makefile

@@ -245,6 +245,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_arr_infer_len \
 	  $(BIN)/test_slice_str_global_zero \
 	  $(BIN)/test_slice_literal_global \
+	  $(BIN)/test_global_arr_elem_field \
 	  $(BIN)/test_dot_str_chained_arg \
 	  $(BIN)/test_dot_slice_arg \
 	  $(BIN)/test_dot_tagged_source \
@@ -589,6 +590,12 @@ $(BIN)/test_slice_literal_global: test/wcc/687_slice_literal_global.c $(BIN)/ww
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_global_arr_elem_field: test/wcc/688_global_arr_elem_field.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_dot_str_chained_arg: test/wcc/692_dot_str_chained_arg.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \