fix(rules): knock prone only on a 5-point overrun margin

PF1e CRB 'Overrun': 'If your maneuver is successful, you move through
the target's space. If your attack exceeds your opponent's CMD by 5 or
more, ... the target is knocked prone.' The engine knocked the target
prone on ANY success. A plain success (margin 0-4) now only moves the
attacker through; prone requires total - CMD >= 5 (_OVERRUN_PRONE_MARGIN).

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

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

src/pf1e_simulator/combat.py

@@ -246,6 +246,7 @@ _PRECISION_IMMUNE_TYPES: frozenset[str] = frozenset({"undead", "construct"})
 _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
 _SAVE_DESCRIPTOR_CONDITIONS = frozenset({
     "fear", "cold", "electricity", "fey", "divination", "poison", "disease",
     "mind_affecting",
@@ -4077,19 +4078,28 @@ class CombatEngine:
         if not m.success:
             self._maneuver_fail(state, target, "overrun", m)
             return
-        target.active_conditions.append(
-            ActiveCondition(
-                condition=CONDITIONS_BY_NAME["prone"],
-                rounds_left=1,
-                source="maneuver: overrun",
+        # PF1e (CRB "Overrun"): a successful maneuver moves you through the
+        # target's square; the target is knocked prone only if the attack
+        # exceeds its CMD by 5 or more.
+        knocked_prone = m.total - m.target_cmd >= _OVERRUN_PRONE_MARGIN
+        if knocked_prone:
+            target.active_conditions.append(
+                ActiveCondition(
+                    condition=CONDITIONS_BY_NAME["prone"],
+                    rounds_left=1,
+                    source="maneuver: overrun",
+                )
             )
-        )
         beyond = self._beyond_cell(state.pos, target.pos)
-        if self._cells_free(self._footprint_at(beyond, state), state):
+        moved_through = self._cells_free(self._footprint_at(beyond, state), state)
+        if moved_through:
             state.pos = beyond
-            outcome = "SUCCESS (prone, moved through)"
-        else:
-            outcome = "SUCCESS (prone)"
+        achieved = ", ".join(
+            label
+            for label, flag in (("prone", knocked_prone), ("moved through", moved_through))
+            if flag
+        )
+        outcome = f"SUCCESS ({achieved})" if achieved else "SUCCESS"
         self._log(
             f"round {self._current_round} {state.combatant.id}: overrun vs "
             f"{target.combatant.id} d20={m.roll}+{m.attacker_cmb}={m.total} "