cmd/w6c+selfhost/wcc: route cgparam/MLET/spill sizes through SSoT

#43 (8e93b31 + 087c85c) routed many sizeof(str) / sizeof(slice)
sites through primtypesize / tyslicesize / ty_*->size, but missed
the cgparam regs-fit, cgparam stack-stitch, cgmlet mixed
scalar+str receive, and vararg slice gather paths in both stages.
A bare #1 bump (str→24B) on top of #43 reds ~60 tests because
those paths still hardcoded 16/24.

Cstage:
- cgen.c:7360-7361 cgmlet: sz0/sz1 → (int)u0->size / (int)u1->size.
- cgen.c:7557 cgparam regs-fit: slice|is_str → (int)pu->size.
- cgen.c:7586 cgparam stack-stitch: same.
- cgen.c:4368 cgcall vararg gather: localoff slice descriptor →
  (int)vsu->size (the cstage twin of cgenexpr.ww:3084).

Wwstage:
- cgendecl.ww:225, :243 cgfnparams: 16 → primtypesize("str"): i32.
- cgenexpr.ww:3084 cgcall vararg gather: 24 → tyslicesize(): i32.

Plus a latent-bug fix at cgenstmt.ww cglet :1031 / :1040: the
str-init and slice-init arms dispatched on size only. Under #1's
str→24, both arms would have fired on a str let (duplicate
MOVQ BX,off+8 + bogus MOVQ CX,off+8). Added isstrtype / isslicetype
kind gates mirroring cstage cgen.c:6439's
`type_isstr(lt) && sz == ty_str->size`. Zero asm change today
because the size constants implicitly disambiguate at 16 vs 24.

Probe with temporary #1 bump (str.size=24) confirms 990_selfhost +
994_w6c_ww go green — the cgen-routing slice for #1 is now
closed. Remaining red under bump is lib/ww/typ.ww's parallel SSoT
seed + stringstest cap*16u64 strides + w6l_ww runtime SIGSEGV;
all tracked separately.

EIGHTBYTES register-count sites (cgen.c:7553-7554, cgendecl.ww:224
/:260) intentionally NOT touched — those are str ABI in-flight
3-reg work (task #34), not slot-width SSoT.

selfhost/cmd/wcc/cgenexpr.ww

@@ -3081,7 +3081,10 @@ fn cgcall(c: *cgen, n: *node) void = {
 				if (nvar > 0) {
 					doff = localadd(c, dname, nvar * esz, nil);
 				};
-				let soff: i32 = localadd(c, sname, 24,
+				// #60: vararg gather builds a {ptr,len,cap} slice
+				// descriptor — route through tyslicesize so #34's
+				// slice-header bump propagates here.
+				let soff: i32 = localadd(c, sname, tyslicesize(): i32,
 				    slicewrap(c, varp.lhs));
 				let aa2: *node = n.list;
 				let kk3: i32 = 0;