w6c+w6c_ww: size-keyed @tagscr — one tagged scratch per slot size (fix #44)

A fn mixing two tagged slot sizes smaller-first (regex compile(): 56B
append-element widen then 64B sret return) hit the #15/#26c rule-7
grow-fatal — the single shared per-fn @tagscr is first-use-sized and
its pinned offset can't grow. Key the scratch by slot size instead:
@tagscr<sz>, one first-use-allocated slot per distinct size, all three
sites (widen-store via_outer, widen-push, N_INDEX tagged-element
assign) funnelled through cg_tagscr_slot / tagscradd in both stages.
Single-size fns emit byte-identical asm to pre-fix (control row pinned
+ hand-cmp'd vs master w6c). 736's tagscr_size_grow_fatal fixture
pinned the now-unreachable fatal; converted to a byte-id succ row.
Runtime rows live in 926_tagscr_sizes_run.

Makefile

@@ -280,6 +280,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_sret_struct_return \
 	  $(BIN)/test_sret_struct_return_run \
 	  $(BIN)/test_tagged_sret_run \
+	  $(BIN)/test_tagscr_sizes_run \
 	  $(BIN)/test_global_sret_run \
 	  $(BIN)/test_sret_narrow_field \
 	  $(BIN)/test_sret_narrow_field_run \
@@ -1171,6 +1172,12 @@ $(BIN)/test_tagged_sret_run: test/wcc/926_tagged_sret_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_tagscr_sizes_run: test/wcc/926_tagscr_sizes_run.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_global_sret_run: test/wcc/940_global_sret_run.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \