feat(engine): 实现阵营技能树系统

- 新增 FactionSkill、FactionSkillTree、FactionSkills 数据模型
- 创建 faction_skills.py 模块处理技能解锁和效果应用
- 技能解锁条件：power >= cost 且前置技能已解锁
- 技能效果：increase_positive_emotion、reduce_conflict、increase_fear、suppress_others
- 添加 4 个技能树测试用例，全部 30 个测试通过

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

engine-python/app/faction_skills.py

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+"""阵营技能树系统 - 允许阵营通过累积能量解锁技能"""
+from typing import List, Optional
+from .models import (
+    WorldState, FactionSkill, FactionSkillTree, FactionSkills
+)
+
+
+def get_default_faction_skills() -> FactionSkills:
+    """返回默认技能树配置"""
+    return FactionSkills(
+        optimists=FactionSkillTree(
+            skills={
+                "festival_boost": FactionSkill(
+                    unlocked=False,
+                    cost=10,
+                    effect="increase_positive_emotion",
+                    requires=[]
+                ),
+                "unity": FactionSkill(
+                    unlocked=False,
+                    cost=20,
+                    effect="reduce_conflict",
+                    requires=["festival_boost"]
+                )
+            }
+        ),
+        fearful=FactionSkillTree(
+            skills={
+                "panic_spread": FactionSkill(
+                    unlocked=False,
+                    cost=10,
+                    effect="increase_fear",
+                    requires=[]
+                ),
+                "control": FactionSkill(
+                    unlocked=False,
+                    cost=20,
+                    effect="suppress_others",
+                    requires=["panic_spread"]
+                )
+            }
+        )
+    )
+
+
+def _check_requires_met(skill_tree: FactionSkillTree, requires: List[str]) -> bool:
+    """检查前置技能是否已解锁"""
+    for req in requires:
+        if req not in skill_tree.skills:
+            return False
+        if not skill_tree.skills[req].unlocked:
+            return False
+    return True
+
+
+def check_and_unlock_skills(state: WorldState) -> List[str]:
+    """检查并解锁技能
+
+    规则：
+    1. faction.power >= skill.cost
+    2. 技能尚未解锁
+    3. 所有 requires 已解锁
+
+    返回：本次解锁的技能列表
+    """
+    unlocked = []
+
+    # 检查乐观派技能
+    for skill_id, skill in state.faction_skills.optimists.skills.items():
+        if skill.unlocked:
+            continue
+        if state.factions.optimists.power < skill.cost:
+            continue
+        if not _check_requires_met(state.faction_skills.optimists, skill.requires):
+            continue
+
+        # 解锁技能
+        skill.unlocked = True
+        state.factions.optimists.power -= skill.cost
+        unlocked.append(f"optimists:{skill_id}")
+
+    # 检查恐惧派技能
+    for skill_id, skill in state.faction_skills.fearful.skills.items():
+        if skill.unlocked:
+            continue
+        if state.factions.fearful.power < skill.cost:
+            continue
+        if not _check_requires_met(state.faction_skills.fearful, skill.requires):
+            continue
+
+        # 解锁技能
+        skill.unlocked = True
+        state.factions.fearful.power -= skill.cost
+        unlocked.append(f"fearful:{skill_id}")
+
+    return unlocked
+
+
+def apply_skill_effects(state: WorldState) -> None:
+    """应用已解锁技能的持续效果
+
+    效果说明：
+    - increase_positive_emotion: 每 tick 提升 town_mood +1
+    - reduce_conflict: 降低恐惧派能量增长（每 tick -1）
+    - increase_fear: 每 tick 降低 town_mood -1
+    - suppress_others: 降低乐观派能量增长（每 tick -1）
+    """
+    # 乐观派技能效果
+    for skill_id, skill in state.faction_skills.optimists.skills.items():
+        if not skill.unlocked:
+            continue
+
+        if skill.effect == "increase_positive_emotion":
+            state.town_mood = min(10, state.town_mood + 1)
+        elif skill.effect == "reduce_conflict":
+            state.factions.fearful.power = max(0, state.factions.fearful.power - 1)
+
+    # 恐惧派技能效果
+    for skill_id, skill in state.faction_skills.fearful.skills.items():
+        if not skill.unlocked:
+            continue
+
+        if skill.effect == "increase_fear":
+            state.town_mood = max(-10, state.town_mood - 1)
+        elif skill.effect == "suppress_others":
+            state.factions.optimists.power = max(0, state.factions.optimists.power - 1)