fix(rules): scale the bull rush push distance with the margin

PF1e (SRD Bull Rush): "your target is pushed back 5 feet. For every 5 by which your attack exceeds your opponent's CMD you can push the target back an additional 5 feet." _bull_rush_push (and the explicit-direction branch) now walk 1 + margin//5 cells, stopping at the first obstacle; a natural-20 auto-success below CMD floors the margin at 0 (base push only). README updated.

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

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

README.md

@@ -1008,7 +1008,11 @@ Règles modélisées :
    creature does not add its Dexterity bonus to its CMD » — la pénalité de
    Dextérité, qui n'est pas un bonus, reste appliquée).
    trip → `prone` ; disarm → `disarmed` (−4 attaque) ; bull_rush → pousse la
-  cible (direction sélectionnable via `Action.direction`) ;
+   cible (direction sélectionnable via `Action.direction`) ; conformance L6
+   (SRD « Bull Rush ») : la poussée vaut 5 ft plus « For every 5 by which
+   your attack exceeds your opponent's CMD you can push the target back an
+   additional 5 feet » — `1 + marge//5` cases, stoppées au premier obstacle
+   (un 20 naturel sous le CMD ne donne que la poussée de base) ;
    grapple → `grappled`/`pinned` ; escape → **une** sauvegarde (bonus statique de la
    compétence `escape_artist` ou CMB, + un d20) contre CMD — une seule tentative,
    sans boucle multi-tour. Conformance L4 (SRD « Combat Maneuvers ») : le 20

src/pf1e_simulator/combat.py

@@ -260,6 +260,7 @@ _AID_ANOTHER_DC = 10  # PF1e: aid another is an attack roll against AC 10
 _AID_ANOTHER_BONUS = 2  # PF1e: a successful aid grants +2 (next attack or AC)
 _RUN_SPEED_FACTOR = 4  # PF1e: running is a full-round action at 4x speed
 _OVERRUN_PRONE_MARGIN = 5  # PF1e: overrun knocks prone only on a 5+ CMB margin
+_BULL_RUSH_EXTRA_PER_MARGIN = 5  # PF1e: +5 ft push per 5 points above CMD
 _SAVE_DESCRIPTOR_CONDITIONS = frozenset({
     "fear", "cold", "electricity", "fey", "divination", "poison", "disease",
     "mind_affecting",
@@ -4165,13 +4166,21 @@ class CombatEngine:
         if not m.success:
             self._maneuver_fail(state, target, "bull_rush", m)
             return
+        # PF1e (SRD Bull Rush): "your target is pushed back 5 feet. For every
+        # 5 by which your attack exceeds your opponent's CMD you can push the
+        # target back an additional 5 feet." A natural-20 auto-success below
+        # CMD still gets only the base 5 ft, hence the margin floor of 0.
+        distance = 1 + max(0, m.total - m.target_cmd) // _BULL_RUSH_EXTRA_PER_MARGIN
         if direction is not None:
-            new_pos = (target.pos[0] + direction[0], target.pos[1] + direction[1])
-            pushed = self._cells_free(self._footprint_at(new_pos, target), target)
-            if pushed:
+            pushed = False
+            for _ in range(distance):
+                new_pos = (target.pos[0] + direction[0], target.pos[1] + direction[1])
+                if not self._cells_free(self._footprint_at(new_pos, target), target):
+                    break
                 target.pos = new_pos
+                pushed = True
         else:
-            pushed = self._bull_rush_push(state, target)
+            pushed = self._bull_rush_push(state, target, distance)
         outcome = "SUCCESS (pushed)" if pushed else "SUCCESS (no space)"
         self._log(
             f"round {self._current_round} {state.combatant.id}: bull rush vs "
@@ -6154,8 +6163,14 @@ class CombatEngine:
         """Size modifier for CMB/CMD: Medium=0, Small=-1, Large=1, etc."""
         return _SIZE_MODIFIERS.get(state.combatant.size.lower(), 0)
 
-    def _bull_rush_push(self, attacker: CombatantState, target: CombatantState) -> bool:
-        """Push target 5ft away from attacker. Returns True if push succeeded."""
+    def _bull_rush_push(
+        self, attacker: CombatantState, target: CombatantState, distance: int
+    ) -> bool:
+        """Push target up to ``distance`` cells away from attacker.
+
+        The push stops at the first obstacle; returns True if the target
+        moved at least one cell.
+        """
         # Calculate direction away from attacker
         ar, ac = attacker.pos
         tr, tc = target.pos
@@ -6166,11 +6181,14 @@ class CombatEngine:
             dr = 1 if dr > 0 else -1
         if dc != 0:
             dc = 1 if dc > 0 else -1
-        new_pos = (tr + dr, tc + dc)
-        if not self._cells_free(self._footprint_at(new_pos, target), target):
-            return False
-        target.pos = new_pos
-        return True
+        moved = False
+        for _ in range(distance):
+            new_pos = (target.pos[0] + dr, target.pos[1] + dc)
+            if not self._cells_free(self._footprint_at(new_pos, target), target):
+                break
+            target.pos = new_pos
+            moved = True
+        return moved
 
     def _beyond_cell(self, attacker_pos: Pos, target_pos: Pos) -> Pos:
         """The 1-cell position on the far side of the target from the attacker."""

tests/test_creature_sizes.py

@@ -534,7 +534,7 @@ class TestBullRushLarge:
         t_state = make_state(target, "e", (3, 3))
         engine = make_engine([a_state, t_state])
         # Push direction: away from attacker (3,1) -> (3,3) means push right
-        result = engine._bull_rush_push(a_state, t_state)
+        result = engine._bull_rush_push(a_state, t_state, 1)
         if result:
             # Target moved 1 cell right
             assert t_state.pos == (3, 4)

tests/test_maneuvers.py

@@ -162,6 +162,38 @@ def test_escape_natural_1_still_fails_above_cmd() -> None:
     assert any(c.condition.name == "grappled" for c in a.active_conditions)
 
 
+def test_bull_rush_push_scales_with_margin() -> None:
+    # SRD Bull Rush: "For every 5 by which your attack exceeds your
+    # opponent's CMD you can push the target back an additional 5 feet."
+    # d20=20+2=22 vs CMD 12: margin 10 -> push 1 + 10//5 = 3 cells.
+    a = make_state(make_combatant("a", attack_bonus=2), "players", (1, 1))
+    b = make_state(make_combatant("b"), "monsters", (1, 2))
+    engine = make_engine([1, 20], [a, b])
+    engine._execute_maneuver(a, b, "bull_rush")
+    assert b.pos == (1, 5)
+
+
+def test_bull_rush_push_stops_at_obstacle() -> None:
+    # Same margin 10 (3 cells) but a creature blocks (1, 4): the push moves
+    # b two cells and stops.
+    a = make_state(make_combatant("a", attack_bonus=2), "players", (1, 1))
+    b = make_state(make_combatant("b"), "monsters", (1, 2))
+    c = make_state(make_combatant("c"), "monsters", (1, 4))
+    engine = make_engine([1, 20], [a, b, c])
+    engine._execute_maneuver(a, b, "bull_rush")
+    assert b.pos == (1, 3)
+
+
+def test_bull_rush_natural_20_below_cmd_pushes_one_cell() -> None:
+    # Natural 20 auto-succeeds even below CMD (bab 20 foe: CMD 30, total 22):
+    # the margin floors at 0, so only the base 5 ft push applies.
+    a = make_state(make_combatant("a", attack_bonus=2), "players", (1, 1))
+    b = make_state(make_combatant("b", attack_bonus=20), "monsters", (1, 2))
+    engine = make_engine([1, 20], [a, b])
+    engine._execute_maneuver(a, b, "bull_rush")
+    assert b.pos == (1, 3)
+
+
 def test_overrun_knocks_prone_and_moves_through() -> None:
     # d20=15+2=17 vs CMD 12: margin 5 -> prone AND moved through (CRB
     # "Overrun": prone only when the attack exceeds CMD by 5 or more).