cgen: load the full header for a wwstage global-dot slice field

let x: []T = g.buf on a module-global struct dispatched only str
fields to the 3-word header load; a []T field fell to the scalar
tail (ptr word only), so len/cap read stale registers and the shape
was byteid-divergent against cstage's TY_STR||TY_SLICE arm (#263).

selfhost/cmd/wcc/cgenexpr.ww

@@ -4242,7 +4242,12 @@ fn cgdot(c: *cgen, n: *syntax.node) void = {
 							cgloadtaggedfield(c, "CX", foff, tsz, true);
 							return;
 						};
-						if (syntax.typeisstr(ftraw)) {
+						// slice joins the str header load (#263: cstage
+						// N_DOT chain arm keys TY_STR||TY_SLICE — the
+						// same 24B {ptr,len,cap}); the scalar tail below
+						// pulled ONLY .ptr, so len(g.buf) read garbage.
+						if (syntax.typeisstr(ftraw)
+						    || syntax.typeisslice(ftraw)) {
 							emitline("\tMOVQ\t");
 							emitdispreg(foff: i64, "CX");
 							emitline(", AX\n");