fix(rules): use Dexterity for the CMB of Tiny and smaller creatures

PF1e (SRD Combat Maneuvers): "Creatures that are size Tiny or smaller use their Dexterity modifier in place of their Strength modifier to determine their CMB." _maneuver_cmb now swaps Str for Dex when the size is Tiny, Diminutive or Fine. True 0-reach and co-occupancy for Tiny-or-smaller stay unmodelled (5-ft grid abstraction), documented in the README.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_maneuvers.py

@@ -7,7 +7,7 @@ it (effectively disarmed). See the plan task-12.
 
 from __future__ import annotations
 
-from pf1e_simulator.combat import CombatantState, CombatEngine
+from pf1e_simulator.combat import _SIZE_MODIFIERS, CombatantState, CombatEngine
 from pf1e_simulator.conditions import CONDITIONS_BY_NAME
 from pf1e_simulator.effects import ActiveCondition
 from pf1e_simulator.grid import Grid
@@ -269,3 +269,34 @@ def test_drag_no_space() -> None:
     assert b.pos == (1, 2)
     assert a.pos == (1, 1)
     assert "no space" in engine._transcript[-1]
+
+
+def _resized(spec: Combatant, size: str, str_score: int, dex_score: int) -> Combatant:
+    return spec.model_copy(
+        update={
+            "size": size,
+            "abilities": spec.abilities.model_copy(
+                update={"str_score": str_score, "dex_score": dex_score}
+            ),
+        }
+    )
+
+
+def test_tiny_or_smaller_cmb_uses_dexterity() -> None:
+    # SRD Combat Maneuvers: "Creatures that are size Tiny or smaller use
+    # their Dexterity modifier in place of their Strength modifier to
+    # determine their CMB." Tiny: str -1, dex +4, bab 2, size -2 -> 2+4-2.
+    engine = make_engine([], [])
+    for size in ("Tiny", "Diminutive", "Fine"):
+        tiny = make_state(_resized(make_combatant("t"), size, 8, 18), "p", (0, 0))
+        expected = 2 + 4 + _SIZE_MODIFIERS[size.lower()]
+        assert engine._maneuver_cmb(tiny) == expected
+
+
+def test_small_or_larger_cmb_uses_strength() -> None:
+    # The Dex substitution stops at Tiny: Small and larger use Strength.
+    engine = make_engine([], [])
+    small = make_state(_resized(make_combatant("s"), "Small", 8, 18), "p", (0, 0))
+    medium = make_state(_resized(make_combatant("m"), "Medium", 8, 18), "p", (0, 1))
+    assert engine._maneuver_cmb(small) == 2 - 1 - 1  # bab + str + size
+    assert engine._maneuver_cmb(medium) == 2 - 1 + 0  # bab + str + size