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.

selfhost/cmd/wcc/cgenexpr.ww

@@ -5679,13 +5679,13 @@ fn cgassign(c: *cgen, n: *node) void = {
 				// scratch slot via cgwidentaggedstore (handles struct /
 				// str / scalar / subset / nullable variants uniformly),
 				// then compute &arr[i] and byte-copy. The scratch
-				// (@tagscr) is reused across all tagged-arr stores in
-				// the function; first-use sizes the slot (#15/#26c).
+				// (@tagscr<sz>) is reused across all same-size tagged-arr
+				// stores in the function; first-use sizes the slot
+				// (#15/#26c, size-keyed by #44).
 				if (elemtn != nil) {
 					if (istaggedtype(c, elemtn)) {
 						let slot_sz: i32 = slotsize(c, elemtn);
-						let scroff: i32 = localadd(c, "@tagscr",
-						    slot_sz, nil);
+						let scroff: i32 = tagscradd(c, slot_sz);
 						// Pre-zero scratch (matches push helper).
 						emitline("\tXORQ\tAX, AX\n");
 						let zz: i32 = 0;