feat(timeline): P2 并行编排——scene 多轨编辑器 + 白模重叠预览

剧情 Timeline P2 前端 + 共享内核（与 SGame 源真同步）：
- ir_core/IR_SCHEMA/样张：scene v0.3 + scene 校验 + 导出 gate(D3)，与 SGame 仓逐字一致
- timeline.js：appendScene 按 authored start 铺多轨 clip(自然重叠预览)，move from 同 actor 跨轨续连(D4)；
  drawStage 改逐 actor 查对话→多人气泡同时计时；导出 _clipDur 纯函数；show() 加 startId 参；常量加 CAMERA_DUR
- scene_edit.js(新)：演出段编辑模态——拖 clip 改 start(吸附 0.1s)、拖右缘改 dur、增删 clip/轨道、
  选中属性条精确编辑、客户端轻量 lint(镜像 validate.py)、▶ 预览此段(复用播放核)
- graph.js：scene 节点(KIND_CN/summary/nodeInner 列轨道)+双击进编辑模态
- form.js：右栏 renderScene 精确数值编辑(轨道/clip 的 start/dur/kind/目标)+打开编辑器按钮
- app.py export：捕获 CompileError 并入 report(scene 被拦时不再 500)
- test_scene.js：离线 10 断言全过(重叠确凿/晚 1.5s 起步/from 续连)；gitignore 忽略本地 _localdemo.db

待浏览器目测拖拽编辑落 IR + 白模重叠演出。

samples/scene_demo.ir.json

@@ -0,0 +1,45 @@
+{
+  "id": "QY_SCENE",
+  "title": "并行演出段演示",
+  "theme": "P2 多轨时间线：A 走、1.5s 后 B 才走，两者时间重叠",
+  "scale": "演示",
+  "roles": [
+    { "slot": "NP1", "name": "神秘剑客", "archetype": "负伤外门高手", "camp": 0 },
+    { "slot": "NP2", "name": "客栈小二", "archetype": "市井路人", "camp": 0 }
+  ],
+  "stage": { "type": "客栈·夜", "point_set": "QY_TLDEMO" },
+  "nodes": [
+    { "id": "n_open", "kind": "narration", "speaker": "P1", "text": "夜色深沉，雨点敲打着客栈的窗棂。两道身影几乎同时动了起来。", "next": "sc_demo" },
+
+    {
+      "id": "sc_demo",
+      "kind": "scene",
+      "next": "n_choice",
+      "tracks": [
+        { "id": "tk_p1", "actor": "P1", "clips": [
+          { "id": "c_p1move", "kind": "move", "start": 0,   "to": "PT_CENTER" },
+          { "id": "c_p1say",  "kind": "dialogue", "start": 0.5, "text": "（这客栈看着冷清，却处处透着古怪……我先到中堂看看。）" }
+        ] },
+        { "id": "tk_np1", "actor": "NP1", "clips": [
+          { "id": "c_np1move", "kind": "move", "start": 1.5, "to": "PT_DOOR" },
+          { "id": "c_np1say",  "kind": "dialogue", "start": 3.0, "text": "阁下留步。在下身负旧伤，想借贵宝地歇一晚，天明即走。" }
+        ] },
+        { "id": "tk_np2", "actor": "NP2", "clips": [
+          { "id": "c_np2say", "kind": "dialogue", "start": 4.5, "text": "二位客官，里边请，小的这就去沏壶热茶！" }
+        ] },
+        { "id": "tk_cam", "role": "camera", "clips": [
+          { "id": "c_cam0", "kind": "camera", "start": 0, "focus": "PT_CENTER", "dur": 6.0 }
+        ] }
+      ]
+    },
+
+    { "id": "n_choice", "kind": "choice", "options": [
+      { "text": "收剑，请他进来歇脚", "goto": "end_peace" },
+      { "text": "拔剑相向，喝令他离开", "goto": "end_fight" }
+    ] }
+  ],
+  "endings": [
+    { "id": "end_peace", "summary": "结义同盟，平安一夜", "grants": [], "result": "success" },
+    { "id": "end_fight", "summary": "刀剑相向，剑客败退雨夜", "grants": [], "result": "fail" }
+  ]
+}