cgen: address module-global str/slice pseudo-field stores via the symbol

A module-global base has no frame slot; treating its local-lookup miss
as offset zero wrote .ptr/.len/.cap at the caller return address. LEAQ
the symbol like the struct-field global arms do. Both stages.

test/lang/global_slice_pseudofield_store_test.ww

@@ -0,0 +1,19 @@
+// A module-global slice header is not a BP-relative local. Writing its
+// pseudo-fields through offset zero overwrites the caller's saved control
+// state instead of the global header.
+
+package global_slice_pseudofield_store_test;
+
+let values: []i64 = [10, 20, 30, 40];
+let replacement: [2]i64 = [70, 80];
+
+@test fn global_header_store() void = {
+	values.ptr = &replacement[0];
+	values.len = 1;
+	values.cap = 2;
+	values.len += 1;
+	assert(values.len == 2);
+	assert(values.cap == 2);
+	assert(values[0] == 70);
+	assert(values[1] == 80);
+};