Add shared battleFighterParse/Serialize, use it everywhere a fighter is (de)serialized

Replaces three independently hand-rolled battlefighter_t JSON readers/
writers (party.jsonc roster loading, BATTLE_START enemy definitions, and
the v1 save-file party member format) with one generic pair in
battlefighter.c: battleFighterParse(fighter, obj) and its inverse
battleFighterSerialize(fighter, doc). Covers team/controller/status
(optional, default ally/player/normal), the 7 base stats (required),
current health/mp (optional, default to max), and an optional abilities
name array.

- party.c: partyInit now parses each roster member via battleFighterParse
  directly, dropping the old partyParseStats/partyParseStatField.
- cutscenebattlestart.c/.h: cutscenebattlestartenemy_t now embeds a full
  battlefighter_t, parsed once at Load time; Start just copies it into the
  enemy pool instead of re-initializing. battletest.jsonc's enemies are
  now flat (dropped the "stats" wrapper) to match.
- saveslotver1.c/.h: rewritten in place to use the shared parser/
  serializer, changing the v1 on-disk field names/shape (no migration -
  acceptable pre-release, documented in saveslotver1.h).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

assets/cutscenes/battletest.jsonc

@@ -4,17 +4,13 @@
       "type": "BATTLE_START",
       "enemies": [
         {
-          "stats": {
-            "attack": 8, "defense": 4, "magic": 0, "speed": 5, "luck": 2,
-            "healthMax": 30, "mpMax": 0
-          },
+          "attack": 8, "defense": 4, "magic": 0, "speed": 5, "luck": 2,
+          "healthMax": 30, "mpMax": 0,
           "abilities": ["fire"]
         },
         {
-          "stats": {
-            "attack": 6, "defense": 3, "magic": 0, "speed": 7, "luck": 2,
-            "healthMax": 20, "mpMax": 0
-          }
+          "attack": 6, "defense": 3, "magic": 0, "speed": 7, "luck": 2,
+          "healthMax": 20, "mpMax": 0
         }
       ]
     }