test(smir): reproduce _projected_ty Subslice drop-glue pruning bug

Add unit test demonstrating that _projected_ty() returns the original
array type for Subslice projections, causing call_edges to miss the
drop_in_place::<[T; M]> edge and reduce_to() to incorrectly prune
the required drop glue.

Add integration test subslice-drop-partial-move.rs that generates
Drop(arr.Subslice(1,3,false)) in the MIR — the exact pattern
described in the Codex review comment.

Author: Stevengre

kmir/src/tests/integration/data/prove-rs/subslice-drop-partial-move.rs

@@ -0,0 +1,20 @@
+// Subslice projection in a Drop terminator's place.
+//
+// `let [first, ..] = arr` moves only `first`; the remaining elements
+// are dropped in place via Drop(arr.Subslice(1, 3, false)).
+// This exercises _projected_ty() resolving the Subslice to the correct
+// [Droppable; 2] type so reduce_to() preserves the drop glue.
+
+struct Droppable(u8);
+
+impl Drop for Droppable {
+    fn drop(&mut self) {}
+}
+
+fn consume(_: Droppable) {}
+
+fn main() {
+    let arr = [Droppable(1), Droppable(2), Droppable(3)];
+    let [first, ..] = arr;
+    consume(first);
+}