cgen: accept an indexed source in aggregate element assignment

The a[i] = src copy loop enumerated ident/field/deref sources; an
N_INDEX rhs fell to the scalar tail and truncated the element. Route
it through the generic place-address funnel. Both stages.

test/lang/aggregate_slice_swap_test.ww

@@ -0,0 +1,26 @@
+// Indexed aggregate assignment must copy from the source element's storage.
+// Sorting uses this three-step swap on slices, so preserving only one word can
+// silently duplicate records while both compiler stages still agree.
+
+package aggregate_slice_swap_test;
+
+type row = struct {
+	key: i64,
+	value: i64,
+	guard: i64,
+};
+
+@test fn distinct_after_swap() void = {
+	let rows: []row = [];
+	append(rows, row { key = 1, value = 11, guard = 111 });
+	append(rows, row { key = 2, value = 22, guard = 222 });
+	let t: row = rows[1];
+	rows[1] = rows[0];
+	rows[0] = t;
+	assert(rows[0].key == 2);
+	assert(rows[0].value == 22);
+	assert(rows[0].guard == 222);
+	assert(rows[1].key == 1);
+	assert(rows[1].value == 11);
+	assert(rows[1].guard == 111);
+};