selfhost/cmd/wcc: route remaining wwstage size dispatch through SSoT

Followup to 8e93b31 (#43).  Audit caught dispatch-gate sites the
sweep missed:

  - cgen.ww letpreintern's `sz == 16` str-let detector — would
    desync from emitletdataw's matching `sz == primtypesize("str"):
    i32` strlit-init branch under #1.
  - cgenstmt.ww cglet str-init MOVQ-BX gate and slice-init MOVQ-BX/CX
    gate (and the belt-and-suspenders N_TSLICE shape check at l.607).
  - cgenexpr.ww cgindex str-element loads (3 sites: globalarr,
    baselocal, generic fallback) and the matching cgassign N_INDEX
    str-element write pair (BX spill + post-index store).

All gates now read `primtypesize("str"): i32` / `tyslicesize(): i32`,
so #1's ty_str.size bump propagates through the same two-place edit
the original commit advertised.  Combined files (w6c/wwdump) updated
in lockstep.

131/131 + 994 + 995 byte-identity green; smoke.combined.ww (lib-only
consumer) emits the same asm pre vs post, confirming the change is
SSoT routing only (no behaviour shift).

selfhost/cmd/wcc/cgen.ww

@@ -1199,7 +1199,10 @@ export fn letpreintern(c: *cgen, file: *node) void = {
 	for (d != nil) {
 		if (d.kind == nkind.N_LET) {
 			let sz: i32 = letemitsize(c, d);
-			if (sz == 16) {
+			// #43: route the str-let gate through primtypesize so
+			// #1 doesn't desync this with emitletdataw's matching
+			// `sz == primtypesize("str"): i32` strlit-init branch.
+			if (sz == primtypesize("str"): i32) {
 				let r: *node = d.rhs;
 				for (r != nil) {
 					if (r.kind != nkind.N_CAST) { break; };