fix(rules): make the run reachability check avoid difficult terrain

PF1e (CRB Movement): "You can't run or charge across difficult
terrain". _execute_run checked the explicit destination against the full
cost field (which routes through difficult terrain at 2x) but descended
with avoid_difficult=True, so a destination beyond rubble was accepted
(ok=True) yet silently truncated at the obstacle. _destination_reachable
now takes the same avoid_difficult constraint (difficult cells removed
from the field; a destination whose footprint touches difficult terrain is
not a legal run endpoint either), so verdict and path agree. The test that
pinned the old silent-stop-short convention now expects
unreachable_destination; README updated.

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

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

README.md

@@ -737,11 +737,15 @@ Règles modélisées :
   (mécanisme `avoid_difficult` de `_descend_path`, symétrique du refus de la
   charge dans `_charge_path_clear`) ; le combattant **avance jusqu'à
   l'obstacle et s'y arrête** (comportement déterministe, retenu plutôt que
-  le rejet de l'action). À l'achèvement, le combattant est `running` jusqu'au
+  le rejet de l'action) **quand la course est auto-dirigée**. À l'achèvement, le combattant est `running` jusqu'au
   début de son prochain tour : il **perd son bonus de Dextérité à la CA**
   (résolu par le même chemin flat-footed/`denies_dex_to_ac` que les
   conditions). Une `destination` hors budget 4× (ou invalide) est rejetée
-  (`unreachable_destination`) ; sans cible ni destination, rejet
+  (`unreachable_destination`) ; une `destination` joignable uniquement en
+  traversant le terrain difficile est **aussi rejetée** — le test de
+  portée (`_destination_reachable`) applique la même contrainte
+  `avoid_difficult` que la descente, donc le verdict et le chemin parcouru
+  sont cohérents (pas d'arrêt silencieux avant la destination) ; sans cible ni destination, rejet
   (`missing_target`). `default_policy` ne choisit pas la course (disponible
   via une politique personnalisée).
 - Pas de placement (5-foot step) : action libre qui déplace le combattant d'une

src/pf1e_simulator/combat.py

@@ -3680,7 +3680,8 @@ class CombatEngine:
         already applies to any other blockage). On completion the combatant
         is ``running`` until the start of its next turn, which denies it its
         Dexterity bonus to AC. An explicit destination that cannot be
-        honoured within the 4x budget is rejected with
+        honoured within the 4x budget WITHOUT crossing difficult terrain (the
+        same constraint the descent applies) is rejected with
         ``unreachable_destination``; with neither target nor destination the
         U1 gate rejects the action with ``missing_target``.
         """
@@ -3697,7 +3698,7 @@ class CombatEngine:
         if action.destination is not None:
             reachable = self._destination_reachable(
                 state, action.destination, speed_factor=_RUN_SPEED_FACTOR,
-                friendly_passable=True,
+                friendly_passable=True, avoid_difficult=True,
             )
             if not reachable:
                 self._log(
@@ -6091,13 +6092,17 @@ class CombatEngine:
         *,
         speed_factor: int = 1,
         friendly_passable: bool = False,
+        avoid_difficult: bool = False,
     ) -> bool:
         """True when destination is passable, unoccupied and within this speed.
 
         ``speed_factor`` widens the budget (run: 4x speed, U5e).
         ``friendly_passable`` routes the cost field around allies' squares
         (move/run may cross them, SRD Movement); the destination itself must
-        still be completely free.
+        still be completely free. ``avoid_difficult`` removes difficult cells
+        from the cost field, mirroring the descent constraint of a run
+        (CRB: "You can't run or charge across difficult terrain") so the
+        reachability verdict and the walked path agree.
         """
         speed_cells = int(
             _speed_ft(state)
@@ -6109,10 +6114,25 @@ class CombatEngine:
             return destination == state.pos
         if not self._cells_free(self._footprint_at(destination, state), state):
             return False
+        if avoid_difficult and self._footprint_touches_difficult(
+            self._footprint_at(destination, state)
+        ):
+            return False
         blocked = self._blocked_hostile(state) if friendly_passable else self._blocked(state)
+        if avoid_difficult:
+            blocked = blocked | self._difficult_cells()
         field = self._grid.reachable(state.pos, speed_cells, blocked)
         return destination in field
 
+    def _difficult_cells(self) -> frozenset[Pos]:
+        """Every passable cell on the grid whose terrain costs more than one."""
+        return frozenset(
+            (row, col)
+            for row in range(self._grid.height)
+            for col in range(self._grid.width)
+            if self._is_difficult_cell((row, col))
+        )
+
     def _walk_steps(
         self, state: CombatantState, path: list[Pos]
     ) -> list[Pos]:

tests/test_aid_and_run.py

@@ -368,12 +368,13 @@ def test_run_stops_before_difficult_terrain_toward_target() -> None:
     assert eng._states[0].pos == (2, 0)
 
 
-def test_run_stops_before_difficult_terrain_to_destination() -> None:
+def test_run_beyond_difficult_terrain_to_destination_is_rejected() -> None:
     """Given: the same rubble row
-    When: hero runs to an explicit destination beyond it
-    Then: the descent stops in front of the rubble (the destination is
-    reachable in budget, but the run cannot cross — it moves up to the
-    obstacle and stops)."""
+    When: hero runs to an explicit destination ONLY reachable across it
+    Then: the destination is rejected (unreachable_destination) — the
+    reachability check uses the same avoid-difficult constraint as the
+    descent (CRB: "You can't run or charge across difficult terrain"), so
+    the verdict and the walked path agree instead of silently stopping short."""
 
     def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
         if state.combatant.id == "hero":
@@ -382,9 +383,28 @@ def test_run_stops_before_difficult_terrain_to_destination() -> None:
 
     grid = _rubble_row_grid(size=8, row=3)
     hero = make_state(_combatant("hero", initiative_mod=2), "players", (0, 0))
-    # The default foe sits at (0, 1), adjacent to the start: the run walks
-    # out of its threatened disk and provokes one (missed) AoO.
-    eng = _duo(policy, [10, 5, 1, 1], round_cap=1, hero=hero, grid=grid)
+    eng = _duo(policy, [10, 5], round_cap=1, hero=hero, grid=grid)
+    result = eng.run()
+    assert eng._states[0].pos == (0, 0)
+    assert "round 1 hero: run rejected (unreachable_destination)" in result.transcript
+
+
+def test_run_destination_in_front_of_difficult_terrain_is_reachable() -> None:
+    """Given: the same rubble row
+    When: hero runs to an explicit destination that avoids the rubble
+    Then: the run honours it (the field routes around difficult cells, the
+    budget is generous — the verdict is positive and the path arrives)."""
+
+    def policy(_engine: CombatEngine, state: CombatantState) -> tuple[Action, ...]:
+        if state.combatant.id == "hero":
+            return (Action(kind="run", destination=(2, 0)),)
+        return _WAIT
+
+    grid = _rubble_row_grid(size=8, row=3)
+    hero = make_state(_combatant("hero", initiative_mod=2), "players", (0, 0))
+    # The default foe at (0, 1) threatens the start: the run provokes one
+    # (missed) AoO on the first step — scripted as a natural 1.
+    eng = _duo(policy, [10, 5, 1], round_cap=1, hero=hero, grid=grid)
     result = eng.run()
     assert eng._states[0].pos == (2, 0)
     assert "round 1 hero: run" in result.transcript