fix(spells): apply a shaken effect when a fear save succeeds

ConditionEffect gains save_partial (JSON 'on_success': condition_name + duration_rounds). On a successful save that would otherwise negate, _apply_condition_effect now imposes the weaker condition (SRD fear.md: panicked on a failed Will save, shaken for 1 round on a success). Extended _metamagic_transform_sub to preserve/double it; extracted _impose_condition helper. Updated fear.json, added failed/successful-save and loader tests in test_spells.py, and the README.

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

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

src/pf1e_simulator/spells.py

@@ -173,6 +173,10 @@ class ConditionEffect:
     condition_name: str
     duration_rounds: int
     save_negates: bool = True
+    # PF1e partial save (SRD spells/fear.md): on a SUCCESSFUL save the target
+    # is not spared outright but suffers a weaker condition instead. Encoded as
+    # (condition_name, duration_rounds); None = a successful save fully negates.
+    save_partial: tuple[str, int] | None = None
 
 
 @dataclass(frozen=True)
@@ -468,6 +472,15 @@ def _parse_recurring(value: Json) -> RecurringDamageSpec:
     )
 
 
+def _parse_save_partial(value: Json) -> tuple[str, int]:
+    data = _as_dict(value)
+    duration = _as_int(data["duration_rounds"])
+    if duration < 1:
+        msg = f"on_success duration_rounds must be >= 1, got {duration}"
+        raise ValueError(msg)
+    return (_as_str(data["condition_name"]), duration)
+
+
 def _parse_effect(data: Json) -> SpellEffect:
     effect = _as_dict(data)
     effect_type = _as_str(effect["type"])
@@ -502,10 +515,13 @@ def _parse_effect(data: Json) -> SpellEffect:
             recurring=recurring,
         )
     if effect_type == "condition":
+        partial_raw = effect.get("on_success")
+        save_partial = None if partial_raw is None else _parse_save_partial(partial_raw)
         return ConditionEffect(
             condition_name=_as_str(effect["condition_name"]),
             duration_rounds=_as_int(effect["duration_rounds"]),
             save_negates=bool(effect.get("save_negates", True)),
+            save_partial=save_partial,
         )
     if effect_type == "heal":
         return HealEffect(