fix(rules): restrict maneuvers to targets within one size category

PF1e (SRD Trip / Bull Rush / Overrun): "You can only [affect] an opponent who is no more than one size category larger than you." _execute_maneuver now rejects trip, bull_rush and overrun against a target two or more size categories larger: no roll, no provoked AoO, logged rejection. A _SIZE_ORDER ranking (Fine..Colossal, unknown -> Medium) backs the check; other maneuvers keep no size cap. README updated.

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

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

src/pf1e_simulator/combat.py

@@ -262,6 +262,7 @@ _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
 _TRIP_RETALIATION_MARGIN = 10  # PF1e: trip failing by 10+ knocks the attacker prone
+_MANEUVER_MAX_SIZE_DIFF = 2  # PF1e: 2+ size categories larger = unmaneuverable
 _SAVE_DESCRIPTOR_CONDITIONS = frozenset({
     "fear", "cold", "electricity", "fey", "divination", "poison", "disease",
     "mind_affecting",
@@ -277,6 +278,24 @@ _SIZE_MODIFIERS: dict[str, int] = {
 # modifier in place of their Strength modifier to determine their CMB.
 _TINY_OR_SMALLER_SIZES: frozenset[str] = frozenset({"tiny", "diminutive", "fine"})
 
+# PF1e size order, smallest first (SRD "Size").
+_SIZE_ORDER: tuple[str, ...] = (
+    "fine", "diminutive", "tiny", "small", "medium", "large", "huge",
+    "gargantuan", "colossal",
+)
+
+# PF1e (SRD Trip / Bull Rush / Overrun): "You can only [affect] an opponent
+# who is no more than one size category larger than you."
+_SIZE_RESTRICTED_MANEUVERS: frozenset[str] = frozenset({"trip", "bull_rush", "overrun"})
+
+
+def _size_rank(size: str) -> int:
+    """Index in the PF1e size order; unknown sizes rank as Medium."""
+    try:
+        return _SIZE_ORDER.index(size.lower())
+    except ValueError:
+        return _SIZE_ORDER.index("medium")
+
 # 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] = {
@@ -3947,6 +3966,18 @@ class CombatEngine:
             "overrun", "dirty_trick", "steal", "reposition", "drag",
         ):
             return
+        # PF1e (SRD Trip / Bull Rush / Overrun): "You can only [affect] an
+        # opponent who is no more than one size category larger than you."
+        # The attempt is illegal outright: no roll, no provoked AoO.
+        if maneuver in _SIZE_RESTRICTED_MANEUVERS and (
+            _size_rank(target.combatant.size) - _size_rank(state.combatant.size)
+            >= _MANEUVER_MAX_SIZE_DIFF
+        ):
+            self._log(
+                f"round {self._current_round} {state.combatant.id}: {maneuver} vs "
+                f"{target.combatant.id} rejected (target too large)"
+            )
+            return
 
         # PF1e (CRB, Combat Maneuver): attempting the maneuver provokes an
         # attack of opportunity from the target — resolved BEFORE the CMB