refactor(model): SpellBook을 SkillBook으로 리네이밍

- 게임 컨셉에 맞게 주문서 → 스킬북 용어 통일
- 관련 모든 참조 일괄 변경

lib/src/core/model/game_state.dart

@@ -21,7 +21,7 @@ class GameState {
     Stats? stats,
     Inventory? inventory,
     Equipment? equipment,
-    SpellBook? spellBook,
+    SkillBook? skillBook,
     ProgressState? progress,
     QueueState? queue,
     SkillSystemState? skillSystem,
@@ -32,7 +32,7 @@ class GameState {
        stats = stats ?? Stats.empty(),
        inventory = inventory ?? Inventory.empty(),
        equipment = equipment ?? Equipment.empty(),
-       spellBook = spellBook ?? SpellBook.empty(),
+       skillBook = skillBook ?? SkillBook.empty(),
        progress = progress ?? ProgressState.empty(),
        queue = queue ?? QueueState.empty(),
        skillSystem = skillSystem ?? SkillSystemState.empty(),
@@ -44,7 +44,7 @@ class GameState {
     Stats? stats,
     Inventory? inventory,
     Equipment? equipment,
-    SpellBook? spellBook,
+    SkillBook? skillBook,
     ProgressState? progress,
     QueueState? queue,
     SkillSystemState? skillSystem,
@@ -57,7 +57,7 @@ class GameState {
       stats: stats,
       inventory: inventory,
       equipment: equipment,
-      spellBook: spellBook,
+      skillBook: skillBook,
       progress: progress,
       queue: queue,
       skillSystem: skillSystem,
@@ -71,7 +71,7 @@ class GameState {
   final Stats stats;
   final Inventory inventory;
   final Equipment equipment;
-  final SpellBook spellBook;
+  final SkillBook skillBook;
   final ProgressState progress;
   final QueueState queue;
 
@@ -93,7 +93,7 @@ class GameState {
     Stats? stats,
     Inventory? inventory,
     Equipment? equipment,
-    SpellBook? spellBook,
+    SkillBook? skillBook,
     ProgressState? progress,
     QueueState? queue,
     SkillSystemState? skillSystem,
@@ -107,7 +107,7 @@ class GameState {
       stats: stats ?? this.stats,
       inventory: inventory ?? this.inventory,
       equipment: equipment ?? this.equipment,
-      spellBook: spellBook ?? this.spellBook,
+      skillBook: skillBook ?? this.skillBook,
       progress: progress ?? this.progress,
       queue: queue ?? this.queue,
       skillSystem: skillSystem ?? this.skillSystem,
@@ -660,26 +660,26 @@ class Equipment {
   }
 }
 
-class SpellEntry {
-  const SpellEntry({required this.name, required this.rank});
+class SkillEntry {
+  const SkillEntry({required this.name, required this.rank});
 
   final String name;
   final String rank; // e.g., Roman numerals
 
-  SpellEntry copyWith({String? name, String? rank}) {
-    return SpellEntry(name: name ?? this.name, rank: rank ?? this.rank);
+  SkillEntry copyWith({String? name, String? rank}) {
+    return SkillEntry(name: name ?? this.name, rank: rank ?? this.rank);
   }
 }
 
-class SpellBook {
-  const SpellBook({required this.spells});
+class SkillBook {
+  const SkillBook({required this.skills});
 
-  final List<SpellEntry> spells;
+  final List<SkillEntry> skills;
 
-  factory SpellBook.empty() => const SpellBook(spells: []);
+  factory SkillBook.empty() => const SkillBook(skills: []);
 
-  SpellBook copyWith({List<SpellEntry>? spells}) {
-    return SpellBook(spells: spells ?? this.spells);
+  SkillBook copyWith({List<SkillEntry>? skills}) {
+    return SkillBook(skills: skills ?? this.skills);
   }
 }