Refactor stack allocator to reuse freed slots

Introduce `free_slots` in `StackAllocator` to keep track of physical
slots that have been freed. When mapping a new virtual slot, the
allocator first attempts to reuse a slot from the `free_slots` vector
before allocating a new one.

This change also includes a new function `collect_scope_locals` that
identifies non-captured local variables defined within a block. After
these expressions are lowered, the allocator reclaims the physical slots
associated with these locals, making them available for reuse by
subsequent blocks.

A new test `test_slot_reuse_non_overlapping_scopes` is added to verify
that distinct scopes correctly reuse stack slots, ensuring optimal stack
usage.

tests/optimizer.rs

@@ -180,3 +180,39 @@ fn test_reproduce_inlining_slot_clash_crash() {
     let res = env.run_script(source);
     assert!(res.is_ok(), "Inlining slot clash triggered: {:?}", res.err());
 }
+
+// ── Slot reuse ────────────────────────────────────────────────────────────────
+
+#[test]
+fn test_slot_reuse_non_overlapping_scopes() {
+    let mut env = Environment::new();
+    env.optimization = true;
+
+    // Two sequential inner blocks each bind a mutable local.
+    // assign prevents the optimizer from inlining the variables away,
+    // so both slots survive into the lowering pass.
+    // x lives only in the first block; once that block ends its slot is free.
+    // y is defined in the second block and must reuse x's slot.
+    // Expected result: the function returns 3 (y = 2+1).
+    let source = r#"
+    (fn []
+      (do
+        (do (def x 1) (assign x (+ x 1)) x)
+        (do (def y 2) (assign y (+ y 1)) y)))
+    "#;
+
+    // Correctness: calling the function must still produce the right value.
+    let result = env
+        .run_script("((fn [] (do (do (def x 1) (assign x (+ x 1)) x) (do (def y 2) (assign y (+ y 1)) y))))")
+        .unwrap();
+    assert_eq!(format!("{}", result), "3");
+
+    // Stack efficiency: with slot reuse the Lambda's stack_size must be 1,
+    // not 2. The dump format is "stack_size: N" in the Lambda metadata line.
+    let dump = env.dump_ast(source).unwrap();
+    assert!(
+        dump.contains("stack_size: 1"),
+        "Non-overlapping scopes must reuse the same slot (expected stack_size: 1). Dump:\n{}",
+        dump
+    );
+}