fix(rules): give Tiny-or-smaller creatures zero reach and square sharing

tests/test_reach.py

@@ -403,3 +403,119 @@ def test_no_aoo_from_adjacent_to_reach_weapon() -> None:
         "round 1 dest: wait",
         "battle over: draw after 1 rounds",
     )
+
+
+# ── S4c: Tiny/Diminutive/Fine natural reach 0 and co-occupancy ───────────────
+
+
+def test_tiny_reach_zero_threatens_nothing() -> None:
+    """S4c: a Tiny creature's natural reach is 0 — it threatens no squares
+    (no AoO) and cannot strike adjacent squares."""
+    tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+    state = make_state(tiny, "players", (3, 3))
+    engine = make_engine([state])
+    assert engine.threatened_squares(state) == frozenset()
+    assert (3, 2) not in engine.threatened_squares(state)
+
+
+def test_tiny_reach_zero_attacks_own_cell_only() -> None:
+    """S4c: a 0-reach Tiny can attack a creature in its OWN cell (distance 0)
+    but not an adjacent one (distance 1)."""
+    tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+    target = make_combatant("target", hp=20, ac=12)
+    tiny_state = make_state(tiny, "players", (3, 3))
+    # Same cell: distance 0 -> the 0-reach attack is usable.
+    foe_same = make_state(target, "monsters", (3, 3))
+    engine_same = make_engine([tiny_state, foe_same])
+    assert engine_same.weapon_for(tiny_state, foe_same) is not None
+    # Adjacent cell: distance 1 (5 ft) -> out of the 0-reach zone.
+    foe_adj = make_state(target, "monsters", (3, 4))
+    engine_adj = make_engine([tiny_state, foe_adj])
+    assert engine_adj.weapon_for(tiny_state, foe_adj) is None
+
+
+def _tiny_attack_policy(engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+    if state.combatant.id == "tiny":
+        return (Action(kind="attack", target_id="target"),)
+    return (Action(kind="wait"),)
+
+
+def test_tiny_reach_zero_hits_creature_in_own_cell() -> None:
+    """S4c: a 0-reach Tiny strikes a creature sharing its cell (the 'entered
+    the square' abstraction) — the attack resolves at distance 0."""
+    tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+    target = make_combatant("target", hp=20, ac=12)
+    tiny_state = make_state(tiny, "players", (3, 3))
+    target_state = make_state(target, "monsters", (3, 3))
+    engine = CombatEngine(
+        ScriptedRng([20, 1, 15, 3]),
+        make_grid(), [tiny_state, target_state],
+        round_cap=1, policy=_tiny_attack_policy,
+    )
+    result = engine.run()
+    assert any(
+        "short sword vs target" in line and "-> HIT" in line
+        for line in result.transcript
+    ), result.transcript
+    assert target_state.hp == 17  # 20 - 1d6(3)
+
+
+def _tiny_move_policy(engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+    if state.combatant.id == "tiny":
+        return (Action(kind="move", destination=(3, 3)),)
+    return (Action(kind="wait"),)
+
+
+def test_tiny_ends_move_in_medium_cell() -> None:
+    """S4c: a Tiny creature may END its move in a cell occupied by a larger
+    creature (co-occupancy)."""
+    tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+    medium = make_combatant("medium", hp=20, ac=15, speed=0)
+    tiny_state = make_state(tiny, "players", (3, 2))
+    medium_state = make_state(medium, "monsters", (3, 3))
+    engine = CombatEngine(
+        ScriptedRng([20, 1, 5]),
+        make_grid(), [tiny_state, medium_state],
+        round_cap=1, policy=_tiny_move_policy,
+    )
+    result = engine.run()
+    assert any("move -> (3,3)" in line for line in result.transcript), result.transcript
+    assert tiny_state.pos == (3, 3)
+
+
+def _medium_move_policy(engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+    if state.combatant.id == "a":
+        return (Action(kind="move", destination=(3, 3)),)
+    return (Action(kind="wait"),)
+
+
+def test_two_medium_cannot_share_cell() -> None:
+    """S4c: two Medium creatures still cannot share a cell — the move onto
+    the occupied cell is rejected."""
+    a = make_combatant("a", speed=30)
+    b = make_combatant("b", hp=20, ac=15, speed=0)
+    a_state = make_state(a, "players", (3, 2))
+    b_state = make_state(b, "monsters", (3, 3))
+    engine = CombatEngine(
+        ScriptedRng([20, 1]),
+        make_grid(), [a_state, b_state],
+        round_cap=1, policy=_medium_move_policy,
+    )
+    result = engine.run()
+    assert any(
+        "move rejected (unreachable_destination)" in line for line in result.transcript
+    ), result.transcript
+    assert a_state.pos == (3, 2)
+
+
+def test_observation_exposes_natural_reach_ft() -> None:
+    """S4c: the combatant snapshot exposes the size's natural reach."""
+    tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+    medium = make_combatant("medium")
+    tiny_state = make_state(tiny, "players", (3, 3))
+    medium_state = make_state(medium, "monsters", (3, 5))
+    engine = make_engine([tiny_state, medium_state])
+    obs = engine._build_observation(tiny_state).to_dict()
+    combatants = {c["id"]: c for c in obs["combatants"]}
+    assert combatants["tiny"]["natural_reach_ft"] == 0
+    assert combatants["medium"]["natural_reach_ft"] == 5