Add regression test for entity.die() during iteration, refs #273

Verifies that die() during grid.entities iteration raises RuntimeError
(iterator invalidation protection), that the safe collect-then-die
pattern works, and that die() properly removes from spatial hash.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/regression/issue_273_die_during_iteration_test.py

@@ -0,0 +1,66 @@
+"""Regression test: entity.die() during iteration raises RuntimeError (#273).
+
+Bug: Calling entity.die() inside a for-loop over grid.entities would
+invalidate the C++ list iterator, causing undefined behavior.
+
+Fix: The EntityCollection iterator captures the collection size at start
+and checks it before each yield. If die() changes the size, RuntimeError
+is raised instead of silently corrupting memory.
+"""
+import mcrfpy
+import sys
+
+def test_die_during_iteration_raises():
+    """entity.die() during iteration raises RuntimeError"""
+    grid = mcrfpy.Grid(grid_size=(20, 20))
+    for i in range(5):
+        mcrfpy.Entity(grid_pos=(i, 0), grid=grid)
+
+    caught = False
+    try:
+        for entity in grid.entities:
+            entity.die()
+    except RuntimeError as e:
+        caught = True
+        assert "changed size during iteration" in str(e), f"Wrong error: {e}"
+
+    assert caught, "Expected RuntimeError when calling die() during iteration"
+    print("  PASS: die_during_iteration_raises")
+
+def test_safe_die_pattern():
+    """Collecting entities first, then dying, works correctly"""
+    grid = mcrfpy.Grid(grid_size=(20, 20))
+    for i in range(5):
+        mcrfpy.Entity(grid_pos=(i, 0), grid=grid)
+
+    assert len(grid.entities) == 5
+
+    # Safe pattern: collect references first
+    to_die = list(grid.entities)
+    for entity in to_die:
+        entity.die()
+
+    assert len(grid.entities) == 0
+    print("  PASS: safe_die_pattern")
+
+def test_die_removes_from_spatial_hash():
+    """entity.die() properly removes from spatial hash"""
+    grid = mcrfpy.Grid(grid_size=(20, 20))
+    entity = mcrfpy.Entity(grid_pos=(5, 5), grid=grid)
+
+    found = grid.entities_in_radius((5.0, 5.0), 1.0)
+    assert len(found) == 1
+
+    entity.die()
+
+    found = grid.entities_in_radius((5.0, 5.0), 1.0)
+    assert len(found) == 0, f"Expected 0 after die(), found {len(found)}"
+    print("  PASS: die_removes_from_spatial_hash")
+
+
+print("Testing entity.die() during iteration (#273)...")
+test_die_during_iteration_raises()
+test_safe_die_pattern()
+test_die_removes_from_spatial_hash()
+print("PASS: all die-during-iteration tests passed")
+sys.exit(0)