fix(rules): convert bane to a 50-ft enemies-only burst

SRD spells/bane.md: enemies within a 50-ft burst centered on the caster
take a -1 penalty on attack rolls and a -1 penalty on saving throws
against fear effects; Will negates. The -1 penalty is approximated with
the shaken condition (-2), matching the existing Doom approximation.

- bane.json: single-target condition -> AreaEffect burst radius_ft 50,
  targets "enemies", ConditionEffect shaken 10 rounds save_negates
  (range/save kept as medium/will for the registry pin)
- combat.py: _metamagic_transform_sub now copies AreaEffect.targets so
  an Extend Spell on an area spell preserves the allies/enemies filter
- tests: TestBaneEnemiesOnly (enemy in burst debuffed, allies/caster
  unaffected, successful Will save negates)

data/spells/bane.json

@@ -8,11 +8,19 @@
   "cast_time": "standard",
   "effects": [
     {
-      "type": "condition",
-      "condition_name": "shaken",
-      "duration_rounds": 10,
-      "save_negates": true
+      "type": "area",
+      "radius_ft": 50,
+      "targets": "enemies",
+      "save_for_half": false,
+      "effects": [
+        {
+          "type": "condition",
+          "condition_name": "shaken",
+          "duration_rounds": 10,
+          "save_negates": true
+        }
+      ]
     }
   ],
-  "description": "Enemies within range take a -1 penalty on attack rolls and saving throws (approximated with the shaken condition, -2). Will negates. Concentration 1 min/level approximated as 10 rounds."
-}
+  "description": "SRD spells/bane.md: enemies within a 50-ft burst centered on the caster take a -1 penalty on attack rolls and a -1 penalty on saving throws against fear effects; Will negates. The -1 penalty is approximated with the shaken condition (-2 on attacks and saves), matching the existing Doom approximation. Duration 1 min/level approximated as 10 rounds."
+}