test: cover scope walks for lambdas inside unpack-target sub-expressions

Two tests added to `unpack__ops.py` that drive the cell-var and
referenced-name walks introduced for `Node::For` / `Node::With` targets:

- `_for_target_lambda` puts a closure-capturing lambda inside the
  index of a for-loop subscript target. Without the walker added in
  `collect_cell_vars_from_unpack_target` /
  `collect_referenced_names_from_unpack_target`, the captured outer
  local would be invisible to scope analysis when nothing else in the
  enclosing function references it.

- `_for_target_tuple_lambda` wraps the same pattern in a nested tuple
  target, exercising the `Tuple` recursion arm in both walkers.

Together these close the codecov gap on the defense-in-depth scope
walks that fire only when a subscript/attribute target's
sub-expression contains a closure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mbeschastn0v

crates/monty/test_cases/unpack__ops.py

@@ -205,3 +205,31 @@ def idx():
 (a[idx()],) = (7,)
 assert a == [7], 'side-effecting index produces correct store'
 assert calls == ['idx'], 'index expression evaluated exactly once at store time'
+
+
+# Lambda inside a `for`-loop subscript-target index — exercises the scope
+# walker that must promote captured outer locals to cell-vars when the only
+# reference to them lives inside the loop target's sub-expressions.
+def _for_target_lambda():
+    x = 1
+    y = [0, 0]
+    for y[(lambda: x)()] in (10, 20):
+        pass
+    return y
+
+
+assert _for_target_lambda() == [0, 20], 'lambda capturing local from inside for-target subscript index'
+
+
+# Same pattern wrapped in a nested tuple target — exercises the Tuple
+# recursion arm in the cell-var / referenced-name unpack-target walkers.
+def _for_target_tuple_lambda():
+    x = 1
+    y = [0, 0]
+    z = ''
+    for y[(lambda: x)()], z in [(10, 'a'), (20, 'b')]:
+        pass
+    return y, z
+
+
+assert _for_target_tuple_lambda() == ([0, 20], 'b'), 'tuple target with lambda-in-subscript-index'