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>

README.md

@@ -976,11 +976,17 @@ Règles modélisées :
   toujours) ; si l'AoO fait passer l'attaquant sous 0 pv, la manœuvre est
   perdue. L'AoO de manœuvre consomme le budget d'AoO du round de
   la cible (Combat Reflexes inclus). Conformance B16 (CRB « Power Attack ») :
-  la pénalité −X de Power Attack actif s'applique **au jet de CMB** de la
+   la pénalité −X de Power Attack actif s'applique **au jet de CMB** de la
-  manœuvre (X = BAB//4 + 1) — la pénalité couvre « les jets d'attaque de mêlée
+   manœuvre (X = BAB//4 + 1) — la pénalité couvre « les jets d'attaque de mêlée
-  et les jets de manœuvres de combat » ; l'escape par compétence n'est pas
+   et les jets de manœuvres de combat » ; l'escape par compétence n'est pas
-  pénalisé (un test de compétence n'est pas un jet de manœuvre).
+   pénalisé (un test de compétence n'est pas un jet de manœuvre). Conformance
-  trip → `prone` ; disarm → `disarmed` (−4 attaque) ; bull_rush → pousse la
+   L1 (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/Diminutive/Fine calculent donc leur CMB avec
+   le modificateur de Dextérité à la place de celui de Force ; lacune
+   documentée : l'allonge nulle et la co-occupation des créatures Tiny et
+   plus petites ne sont pas modélisées (abstraction à la grille de 5 ft).
+   trip → `prone` ; disarm → `disarmed` (−4 attaque) ; bull_rush → pousse la
   cible (direction sélectionnable via `Action.direction`) ;
   grapple → `grappled`/`pinned` ; escape → **une** sauvegarde (bonus statique de la
   compétence `escape_artist` ou CMB, + un d20) contre CMD — une seule tentative,

src/pf1e_simulator/combat.py

@@ -271,6 +271,10 @@ _SIZE_MODIFIERS: dict[str, int] = {
     "medium": 0, "large": 1, "huge": 2, "gargantuan": 4, "colossal": 8,
 }
 
+# PF1e (SRD Combat Maneuvers): creatures Tiny or smaller use their Dexterity
+# modifier in place of their Strength modifier to determine their CMB.
+_TINY_OR_SMALLER_SIZES: frozenset[str] = frozenset({"tiny", "diminutive", "fine"})
+
 # PF1e natural reach for tall creatures (simplification: no long/quadruped
 # distinction). Unknown sizes fall back to 5 ft (the historical behavior).
 _SIZE_NATURAL_REACH_FT: dict[str, int] = {
@@ -4033,9 +4037,17 @@ class CombatEngine:
         )
 
     def _maneuver_cmb(self, state: CombatantState) -> int:
+        # 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."
+        ability_mod = (
+            state.combatant.abilities.dex_mod
+            if state.combatant.size.lower() in _TINY_OR_SMALLER_SIZES
+            else state.combatant.abilities.str_mod
+        )
         return (
             state.combatant.cmb
-            or (state.combatant.bab + state.combatant.abilities.str_mod + self._size_mod(state))
+            or (state.combatant.bab + ability_mod + self._size_mod(state))
         )
 
     def _maneuver_cmd(self, target: CombatantState) -> int:

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