docs: initialize rewrite plan and freeze backend surfaces

backend-rewrite-tasklist/03_M4_STORY_AND_GAMEPLAY.md

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+# M4：story action 与 gameplay reducer 任务清单
+
+## 1. SpacetimeDB gameplay 表
+
+- [ ] 设计 `story_session`
+- [ ] 设计 `story_event`
+- [ ] 设计 `npc_state`
+- [ ] 设计 `quest_record`
+- [ ] 设计 `inventory_slot`
+- [ ] 设计 `treasure_record`
+- [ ] 设计 `battle_state`
+- [ ] 设计 `player_progression`
+- [ ] 设计 `chapter_progression`
+
+## 2. 核心 reducer
+
+- [ ] 设计 `resolve_story_action`
+- [ ] 设计 `continue_story`
+- [ ] 设计 `begin_story_session`
+- [ ] 设计 `sync_runtime_snapshot_projection`
+- [ ] 设计 `apply_quest_signal`
+- [ ] 设计 `apply_inventory_mutation`
+- [ ] 设计 `resolve_npc_interaction`
+- [ ] 设计 `resolve_treasure_interaction`
+- [ ] 设计 `resolve_combat_action`
+- [ ] 设计 `update_progression_state`
+
+## 3. 迁移模块规则
+
+- [ ] 迁移 `story`
+- [ ] 迁移 `combat`
+- [ ] 迁移 `inventory`
+- [ ] 迁移 `npc`
+- [ ] 迁移 `progression`
+- [ ] 迁移 `quest`
+- [ ] 迁移 `runtime-item`
+- [ ] 迁移 `runtime` 的状态归一化规则
+
+## 4. 兼容接口
+
+- [ ] 兼容 `POST /api/runtime/story/actions/resolve`
+- [ ] 兼容 `GET /api/runtime/story/state/:sessionId`
+- [ ] 兼容 `POST /api/runtime/story/initial`
+- [ ] 兼容 `POST /api/runtime/story/continue`
+
+## 5. ViewModel 兼容
+
+- [ ] 兼容当前 `RuntimeStoryActionResponse`
+- [ ] 兼容当前 `RuntimeStoryOptionView`
+- [ ] 兼容当前 `interaction` 元数据
+- [ ] 兼容当前 battle / toast / patch 响应结构
+- [ ] 兼容当前 `currentStory` 回填逻辑
+
+## 6. 阶段验收
+
+- [ ] 当前前端 story 选项点击后可走新后端闭环
+- [ ] NPC / quest / treasure / combat 主循环行为不回退
+- [ ] `story state` 恢复链可用
+- [ ] 旧 Node 版 story route 回归用例完成平移