Add tests pinning EntityCollection semantics and O(1) indexing (#329)

Pre-change TDD tests for the std::list -> std::vector container swap:

- tests/unit/entity_collection_mutation_test.py pins observable behavior:
  positive/negative indexing, out-of-range IndexError, slicing (returns a
  fresh list; slice wrappers are not cache-identity), in-order iteration,
  RuntimeError on size change during iteration, and grid.step() immunity to
  mid-step entity self-removal.
- tests/regression/issue_329_entity_index_perf_test.py guards against a
  regression to O(n) indexed access / O(n^2) full-scan.

Both pass against the pre-swap std::list implementation, locking in the
public contract before the internal container change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/entity_collection_mutation_test.py

@@ -0,0 +1,190 @@
+#!/usr/bin/env python3
+"""
+Semantics test for issue #329: grid.entities backed by std::vector.
+
+Pins the observable behavior of the EntityCollection sequence after the
+container swap from std::list to std::vector:
+
+  * indexing (positive and negative)
+  * out-of-range IndexError
+  * slicing returns a plain list
+  * iteration yields every element in order
+  * mutation during iteration raises RuntimeError (size-guard / generation
+    check) -- iteration is index-based, so this is well-defined and can
+    never corrupt memory even in the remove+add "same size" case
+  * grid.step() turn processing is immune to entity death mid-step
+    (it snapshots into a vector internally)
+
+These behaviors match the pre-#329 std::list implementation; the swap is a
+purely internal container change.
+"""
+
+import mcrfpy
+import sys
+
+results = []
+
+
+def check(name, cond):
+    results.append((name, bool(cond)))
+    print(("  ok  " if cond else " FAIL") + " : " + name)
+
+
+def make_grid(name, count):
+    scene = mcrfpy.Scene(name)
+    grid = mcrfpy.Grid(grid_size=(50, 50), pos=(0, 0), size=(400, 400))
+    scene.children.append(grid)
+    ents = grid.entities
+    made = []
+    for i in range(count):
+        e = mcrfpy.Entity((i, i))
+        ents.append(e)
+        made.append(e)
+    return grid, ents, made
+
+
+def test_indexing():
+    grid, ents, made = make_grid("m_idx", 5)
+    check("len == 5", len(ents) == 5)
+    check("positive index [0] is first", ents[0] is not None)
+    # identity is preserved through the cache: the same Entity comes back
+    check("index preserves identity", ents[2] is made[2])
+    check("negative index [-1] == [4]", ents[-1] is made[4])
+    check("negative index [-5] == [0]", ents[-5] is made[0])
+
+
+def test_out_of_range():
+    grid, ents, made = make_grid("m_oob", 3)
+    got = False
+    try:
+        _ = ents[3]
+    except IndexError:
+        got = True
+    check("index 3 raises IndexError", got)
+
+    got = False
+    try:
+        _ = ents[-4]
+    except IndexError:
+        got = True
+    check("index -4 raises IndexError", got)
+
+
+def _same_entity(a, b):
+    # Slicing returns fresh Entity wrappers (it does not consult the identity
+    # cache), so compare the underlying entity by its distinguishing position
+    # rather than Python object identity. Each entity i was created at (i, i).
+    return a.pos.x == b.pos.x and a.pos.y == b.pos.y
+
+
+def test_slicing():
+    grid, ents, made = make_grid("m_slice", 6)
+    sl = ents[1:4]
+    check("slice returns list", isinstance(sl, list))
+    check("slice length correct", len(sl) == 3)
+    check("slice contents in order",
+          _same_entity(sl[0], made[1]) and _same_entity(sl[1], made[2])
+          and _same_entity(sl[2], made[3]))
+
+    sl2 = ents[::2]
+    check("extended slice length", len(sl2) == 3)
+    check("extended slice contents",
+          _same_entity(sl2[0], made[0]) and _same_entity(sl2[1], made[2])
+          and _same_entity(sl2[2], made[4]))
+
+    sl3 = ents[-2:]
+    check("negative slice contents",
+          len(sl3) == 2 and _same_entity(sl3[0], made[4])
+          and _same_entity(sl3[1], made[5]))
+
+
+def test_iteration_order():
+    grid, ents, made = make_grid("m_iter", 5)
+    seen = [e for e in ents]
+    check("iteration visits all", len(seen) == 5)
+    check("iteration in order",
+          all(seen[i] is made[i] for i in range(5)))
+
+
+def test_mutation_during_iteration():
+    grid, ents, made = make_grid("m_mut", 5)
+    # Removing during iteration changes size -> well-defined RuntimeError.
+    raised = False
+    try:
+        for e in ents:
+            ents.remove(e)
+    except RuntimeError:
+        raised = True
+    check("remove during iteration raises RuntimeError", raised)
+
+    # append during iteration also raises (size changed).
+    grid2, ents2, made2 = make_grid("m_mut2", 3)
+    raised = False
+    try:
+        for e in ents2:
+            ents2.append(mcrfpy.Entity((30, 30)))
+            break_now = len(ents2) > 6  # guard against infinite loop if no raise
+            if break_now:
+                break
+    except RuntimeError:
+        raised = True
+    check("append during iteration raises RuntimeError", raised)
+
+
+def test_step_immune_to_death():
+    # grid.step() must snapshot internally so an entity dying mid-step
+    # (removing itself from the collection) cannot invalidate the loop.
+    scene = mcrfpy.Scene("m_step")
+    grid = mcrfpy.Grid(grid_size=(20, 20), pos=(0, 0), size=(200, 200))
+    scene.children.append(grid)
+    ents = grid.entities
+
+    removed = []
+    for i in range(5):
+        e = mcrfpy.Entity((i, 0))
+        e.turn_order = 1
+        # PATH behavior with an empty path resolves to DONE on the first
+        # step, which fires the `step` callback deterministically.
+        e.set_behavior(int(mcrfpy.Behavior.PATH))
+
+        def on_step(trigger, data, _e=e):
+            # On its turn, remove itself from the grid mid-step.
+            try:
+                grid.entities.remove(_e)
+                removed.append(_e)
+            except ValueError:
+                pass
+
+        e.step = on_step
+        ents.append(e)
+
+    before = len(ents)
+    # Must not crash even though each callback mutates the live collection
+    # while step() is iterating its internal snapshot.
+    grid.step(1)
+    after = len(ents)
+    check("grid.step() survived mid-step self-removal (len %d -> %d, "
+          "%d removed)" % (before, after, len(removed)), True)
+    # If callbacks fired (expected), entities were actually removed.
+    check("mid-step removals took effect", after == before - len(removed))
+
+
+def main():
+    test_indexing()
+    test_out_of_range()
+    test_slicing()
+    test_iteration_order()
+    test_mutation_during_iteration()
+    test_step_immune_to_death()
+
+    failed = [n for n, ok in results if not ok]
+    if failed:
+        print("\nFAIL: %d checks failed: %s" % (len(failed), ", ".join(failed)))
+        return False
+    print("\nPASS: all %d checks passed" % len(results))
+    return True
+
+
+if __name__ == "__main__":
+    ok = main()
+    sys.exit(0 if ok else 1)