feat: add direct edit handles behind directEdit flag

Steps 1-3 of the main map road interval editor.

EditSession keeps the command stack, undo/redo and previewSeq arbitration as
pure logic with no OpenLayers reference, so all of it is unit-tested in node.
Pointer displacement converts to meters through EPSG:4326 and spherical
distance: treating a 3857 delta as meters desyncs the geometry from the cursor
by 1/cos(latitude). Handle drags project onto the axis the manifest declares
and clamp to its range, so the client never writes a coordinate into a road
polygon.

All of it sits behind a directEdit flag that defaults to off. With the flag off
the workbench requests no manifest, creates no extra source and registers no
interaction, so behaviour matches main.

The ol-ext probe passed its three gates but is not adopted for road handles.
Transform translates by the raw pointer delta, so a handle detaches from its
clamped constraint value: a drag reading -24.1 m produced a draft of -5.4 m.
Production needs the handle position derived from the constraint instead, which
means owning the position update, so native OL PointerInteraction will carry
the drag. ol-ext stays out of package.json; the probe is kept as a manual
harness. Reserve handles are unreachable with the current solver, recorded in
research/ rather than worked around.

Also names the dead backend when an API response is empty, instead of
surfacing "Unexpected end of JSON input" from response.json().

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

workbench/client/src/edit/selection.test.ts

@@ -0,0 +1,115 @@
+import { describe, expect, it } from 'vitest';
+import { disabledReasonOf, handlesForSegment, reservesForSegment } from './selection';
+import type { ConstraintKind, EditHandle, HandleManifest, SemanticAnchor } from './types';
+
+function handle(handleId: string, kind: ConstraintKind, anchor: SemanticAnchor, editable = true): EditHandle {
+  return {
+    handleId,
+    kind,
+    anchor,
+    position: [116.397, 39.908],
+    axisAzimuth: 90,
+    value: { current: 0, min: -5, max: 5, unit: 'meter' },
+    affects: [],
+    editable,
+    ...(editable ? {} : { disabledReason: '该道路全部位于路口保留区，请进入 JunctionTools 编辑。' }),
+  };
+}
+
+const roadInterval = (roadId: string): SemanticAnchor => ({
+  type: 'road-interval',
+  roadId,
+  startStation: 0.15,
+  endStation: 0.85,
+  side: 'left',
+});
+
+// Two segments, plus a junction handle the main map must never render.
+const manifest: HandleManifest = {
+  schema: 'road-edit-handles/v1',
+  revisionId: 'rev-0001',
+  previewSeq: 0,
+  handles: [
+    handle('h:edge:a', 'road-edge-offset', roadInterval('road:way/1:forward')),
+    handle('h:sidewalk:a', 'road-sidewalk-width', roadInterval('road:way/1:forward')),
+    handle('h:edge:b', 'road-edge-offset', roadInterval('road:way/2:forward')),
+    handle('h:blocked:a', 'road-lane-divider', roadInterval('road:way/1:forward'), false),
+    handle('h:approach', 'junction-approach-width', { type: 'junction-approach', nodeId: '9', segmentId: 'seg:1' }),
+    handle('h:corner', 'junction-corner-radius', {
+      type: 'junction-corner',
+      nodeId: '9',
+      incomingRoadId: 'seg:1',
+      outgoingRoadId: 'seg:2',
+    }),
+  ],
+  reserves: [
+    { nodeId: '9', roadId: 'seg:1', fromStation: 0, toStation: 0.15 },
+    { nodeId: '10', roadId: 'seg:1', fromStation: 0.85, toStation: 1 },
+    { nodeId: '11', roadId: 'seg:2', fromStation: 0, toStation: 0.3 },
+  ],
+};
+
+const segmentOf = (roadId: string) =>
+  ({ 'road:way/1:forward': 'seg:1', 'road:way/2:forward': 'seg:2' })[roadId] ?? undefined;
+
+describe('handlesForSegment', () => {
+  it('keeps only the selected segment', () => {
+    expect(handlesForSegment(manifest, 'seg:1', segmentOf).map((item) => item.handleId)).toEqual([
+      'h:edge:a',
+      'h:sidewalk:a',
+      'h:blocked:a',
+    ]);
+    expect(handlesForSegment(manifest, 'seg:2', segmentOf).map((item) => item.handleId)).toEqual(['h:edge:b']);
+  });
+
+  it('never renders junction kinds on the main map', () => {
+    // Reserve interiors belong to JunctionTools; offering a second way to edit
+    // them here is exactly the ownership split design.md forbids.
+    const kinds = handlesForSegment(manifest, 'seg:1', segmentOf).map((item) => item.kind);
+    expect(kinds).not.toContain('junction-approach-width');
+    expect(kinds).not.toContain('junction-corner-radius');
+  });
+
+  it('keeps disabled handles so the boundary stays explainable', () => {
+    const blocked = handlesForSegment(manifest, 'seg:1', segmentOf).find((item) => !item.editable);
+    expect(blocked?.handleId).toBe('h:blocked:a');
+  });
+
+  it('returns nothing without a manifest or a selection', () => {
+    expect(handlesForSegment(null, 'seg:1', segmentOf)).toEqual([]);
+    expect(handlesForSegment(manifest, null, segmentOf)).toEqual([]);
+  });
+
+  it('drops handles whose road no longer resolves to a segment', () => {
+    expect(handlesForSegment(manifest, 'seg:1', () => undefined)).toEqual([]);
+  });
+});
+
+describe('reservesForSegment', () => {
+  it('returns only that segment’s reserves', () => {
+    expect(reservesForSegment(manifest, 'seg:1').map((item) => item.nodeId)).toEqual(['9', '10']);
+    expect(reservesForSegment(manifest, 'seg:2').map((item) => item.nodeId)).toEqual(['11']);
+  });
+
+  it('returns nothing without a manifest or a selection', () => {
+    expect(reservesForSegment(null, 'seg:1')).toEqual([]);
+    expect(reservesForSegment(manifest, null)).toEqual([]);
+  });
+});
+
+describe('disabledReasonOf', () => {
+  it('says nothing for an editable handle', () => {
+    expect(disabledReasonOf(handle('h', 'road-edge-offset', roadInterval('road:way/1:forward')))).toBeUndefined();
+  });
+
+  it('passes the server reason through', () => {
+    const blocked = handle('h', 'road-edge-offset', roadInterval('road:way/1:forward'), false);
+    expect(disabledReasonOf(blocked)).toContain('JunctionTools');
+  });
+
+  it('always gives some reason, even when the server omitted one', () => {
+    const blocked = { ...handle('h', 'road-edge-offset', roadInterval('road:way/1:forward'), false) };
+    delete blocked.disabledReason;
+    expect(disabledReasonOf(blocked)).toContain('路口保留区');
+  });
+});