w6c+w6c_ww: store full slice header for struct-literal slice fields (fix #24)

cg_structlit_fill / cgstructlitfill had a TY_STR arm that stored all
three header words (ptr@+0, len@+8, cap@+16) but no TY_SLICE arm, so a
slice field in a struct literal `cl{ items = b, n = .. }` fell through to
the generic scalar tail and stored only the ptr word — the field's .len
and .cap read 0. str fields (the same 24B {ptr,len,cap} shape) worked;
slice fields silently dropped two words.

Both stages emitted IDENTICAL wrong asm, so the 990-997 byte-id gate was
green on both-wrong; runtime readback is the only correctness net. Same
is_str/is_slice discrimination gap as #10 part-b, here in the
struct-literal field-init path.

A slice is the same 24B header shape as str, so widen the str arm's
guard to TY_STR || TY_SLICE (cstage) / isstrtype || isslicetype
(wwstage) and let a slice ride the already-correct 3-word store. The
TAGGED arm stays ordered before it, so a nullable/tagged slice
(TY_TAGGED) still routes to the widener, not the 3-word store.

Test 689 (table-driven, runtime readback + dual-stage asm byte-id):
slice .len/.cap/.ptr, a scalar field beside/before the slice, a slice at
a non-zero field offset, two slice fields, and a str field beside a
slice (str-arm regression pin). 33/33 ok.

selfhost/cmd/wwdump/main.combined.ww

@@ -19519,12 +19519,15 @@ fn cgstructlitfill(c: *cgen, si: *structinfo, lit: *node,
                         fi = nil;
                     } else if (callwhole) {
                         fi = nil;
-                    } else if (isstrtype(c, fi.tnode)) {
+                    } else if (isstrtype(c, fi.tnode) || isslicetype(c, fi.tnode)) {
                         // str IS []u8: 3-word field (ptr,len,cap).
                         // cgexpr leaves AX/BX/CX; for non-BP modes
                         // the dst base goes in DX to dodge BX=len /
                         // CX=cap (the generic store reloads BX, which
-                        // would clobber len) (#1/Phase 3).
+                        // would clobber len) (#1/Phase 3). A slice is
+                        // the same 24B {ptr,len,cap} shape, so it rides
+                        // this arm; without it the generic scalar tail
+                        // stored only the ptr word (#24).
                         cgexpr(c, fieldnode.lhs);
                         if (mode == 0) {
                             emitline("\tMOVQ\tAX, ");