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).

test/wcc/data/globaldot_slicefield_hdr/case.ww

@@ -0,0 +1,25 @@
+//ww:run
+// A module-global struct's slice field read BY VALUE must load the
+// full 24B {ptr,len,cap} header. The pre-fix wwstage global-dot arm
+// dispatched only str fields to the header load; a []T field fell to
+// the scalar tail (ptr word only) and len(x) read stale registers.
+package main;
+
+type box = struct {
+	buf: []i32,
+	name: str,
+};
+
+let g: box = box{};
+
+export fn main() int = {
+	let arr: [4]i32 = [10, 20, 30, 40];
+	g.buf = arr[:];
+	g.name = "globals";
+	let x: []i32 = g.buf;
+	if (len(x) != 4) { return 1; };
+	if (x[2] != 30) { return 2; };
+	let s: str = g.name;
+	if (len(s) != 7) { return 3; };
+	return 0;
+};