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>

data/spells/fear.json

@@ -17,10 +17,11 @@
           "type": "condition",
           "condition_name": "panicked",
           "duration_rounds": 4,
-          "save_negates": true
+          "save_negates": true,
+          "on_success": { "condition_name": "shaken", "duration_rounds": 1 }
         }
       ]
     }
   ],
-  "description": "SRD spells/fear.md: each living creature in the 30-ft. cone becomes panicked unless it succeeds on a Will save (a successful save makes it shaken for 1 round instead). Modeled as panicked for 4 rounds on a failed save; the shaken-on-success partial needs partial-save support (documented limitation). The SRD range is 30 ft. - modeled as the closest category (close)."
+  "description": "SRD spells/fear.md: each living creature in the 30-ft. cone becomes panicked unless it succeeds on a Will save; a creature that succeeds on the save is instead shaken for 1 round (partial save). Modeled as panicked for 4 rounds on a failed save, shaken for 1 round on a successful save. The SRD range is 30 ft. - modeled as the closest category (close)."
 }