feat: make crosswalk and stop-line offsets solvable

Step 1 of the control-marking task, and deliberately server-only: no handle is
drawn yet. This project already shipped a range handle for `profile.interval`,
which compileGeometry ignores, so the control dragged and changed nothing. The
consumer comes first now.

Two kinds join the taxonomy — `junction-crosswalk-inset` and
`junction-stop-line-offset`, both on the existing `junction-approach` anchor. The
solver writes them onto the approach entry, `applyDirectJunctionPlans` carries
them onto the compiled approach, and `compileControlMarkings` reads them in place
of the module constants it used for every junction. They move markings without
reshaping the junction, so unlike width and cutback they deliberately do not
trigger a boundary recompute.

`applyJunctionConstraint` becomes an explicit switch. Its trailing `else` had
meant every kind that was not approach-width fell through to the cutback
validator, so a new kind would have been silently validated and written as a
cutback. The same non-exhaustive shape in the test fixture's `valueFor` is fixed
the same way, and now throws for an unnamed kind rather than answering with a
corner radius.

design.md's taxonomy is updated with it — a test asserts the two cannot drift,
which is what caught the omission.

Measured on a 41-road workspace with 8 crossings: both constraints change their
marking geometry, neither drags the other, and out-of-range blocks instead of
clamping. That measurement is not in the suite: the synthetic junction resolves
`junction_inset_m` to 0 because its crossing never binds to a plan, and the
committed OSM fixture has no crossings at all. The tests assert the wiring the
handles will depend on — values reaching the approach entry, distinct branches,
blocking diagnostics — and the gap is recorded in the test itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.trellis/tasks/08-28-junction-control-offsets/prd.md

@@ -0,0 +1,91 @@
+# 路口控制标线偏移：斑马线与停止线可拖
+
+父任务：`.trellis/tasks/08-26-direct-manipulation-road-editor`。
+这是「更多元素可直接操纵」方向的第一个元素类型。
+
+## 为什么先做这一类
+
+`08-26-direct-edit-map-editor` 交付了完整的直接操纵链路——
+manifest → 命中 → 拖拽 → 投影 → 钳位 → ghost → 防抖 → 乱序仲裁 → 预览 → 保存 → 撤销。
+这条链路**不认识具体元素**，只认 `kind + anchor + axis + value`。所以加一类元素
+本应是「定义一个 kind + 一个写入器」，而不是造新子系统。
+
+选斑马线/停止线打头，因为可行性已实测确认（2026-08-28）：
+
+```js
+611  model = modelWithDirectEditProfiles(model, options.directEdit);
+613  const junctionPlans = compileJunctionPlans(model, options, diagnostics);
+614  applyDirectJunctionPlans(junctionPlans, options.directEdit);   // 直接编辑已合并
+768  const controls = compileControlMarkings(model, lanes, diagnostics, junctionPlans);
+1299   const plan = junctionPlans.get(junctionNodeId);              // 已按路口取到 plan
+```
+
+junction 级约束**已经能流到控制标线的生成**，管线是通的。挡路的只是模块级常量：
+
+```js
+const STOP_LINE_OFFSET_METERS = 2.7;
+const CROSSWALK_JUNCTION_INSET_METERS = 1.5;
+```
+
+它们全局一个值、不按进口区分。把常量换成「先查进口配置，没有再用默认」即可。
+
+这与区间那件事性质不同：区间要求横断面**沿路变化**，是几何管线重构；这里只是
+在已经拿到手的 `plan` 上多读一个字段。
+
+## 强制顺序：编译器优先，UI 最后
+
+`08-26-direct-edit-map-editor` 交付了一个拖了没有任何效果的区间手柄，因为
+`compileGeometry()` 从不读 `profile.interval`——UI 先于消费者建成。
+见 `research/interval-not-applied.md`。
+
+本任务反过来：**第 1 步不产出任何 UI**，只证明编译器真的消费这个参数。
+第 1 步的门禁不过就停，不进入第 2、3 步。
+
+## 范围
+
+### 1. 服务端打通并证明（无 UI）
+
+- 新增两个约束 kind：`junction-crosswalk-inset`、`junction-stop-line-offset`，
+  锚点均为 `junction-approach`（求解器已建模该锚点）。
+- `native-road-edits.js`：schema 校验与取值范围。
+- `direct-edit-solver.js`：`applyJunctionConstraint()` 把值写进该进口的 plan 条目。
+- `native-road.js`：`compileControlMarkings()` 读进口配置，缺省时退回既有常量。
+
+### 2. 手柄目标泛化
+
+`EditHandle` 声明「一次手势写到哪里」，客户端在提交处分岔。
+验收标准是：**加第 N+1 种元素时不需要修改任何已有<E5B7B2><E69C89>柄代码**。
+
+### 3. 手柄与拖拽
+
+选中路口后斑马线、停止线上出现手柄，沿进口方向拖动调整其距路口的距离。
+复用既有链路，不新增预览或保存机制。
+
+## 不做
+
+- 不做红绿灯（属「有位姿的实体资产」，需要不同的写入器与相对锚点）
+- 不做箭头（需先定交互语义：拖箭头应改行驶动作，而非移动图标）
+- 不做区间生效，方块手柄继续隐藏
+- 不改 `STOP_LINE_MAX_APPROACH_DISTANCE_METERS` 等与放置**是否成立**有关的判据，
+  只改放置**位置**
+
+## 验收标准
+
+### 第 1 步（门禁，不过则停）
+
+- [ ] 带 `junction-crosswalk-inset` 约束编译出的斑马线坐标，与不带时**不同**
+- [ ] 带 `junction-stop-line-offset` 约束编译出的停止线坐标，与不带时**不同**
+- [ ] 无约束时输出与当前 main **逐字节相同**（fixture 基线无 diff）
+- [ ] 越界取值产生阻塞诊断，不静默钳制
+- [ ] 约束经保存与重新加载后状态为 `exact`
+
+### 第 2、3 步
+
+- [ ] 选中路口后斑马线与停止线出现手柄，拖动可改变其距路口距离
+- [ ] 手柄位置由钳位值反算，拖到极限停在边界
+- [ ] 新增元素类型未修改既有手柄代码（以 diff 佐证）
+- [ ] `npm run format:check`、`npm run test`、`npm run test:client`、`npm run test:client:unit`、`npm run build` 全绿
+
+## 顺序依赖
+
+无前置。后继：红绿灯位姿与箭头语义，二者都依赖第 2 步的目标泛化。