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/probe/ol-ext-transform/fixture.ts

@@ -0,0 +1,61 @@
+// Synthetic road for the probe, so the page runs without importing OSM or
+// touching the workbench API. Geometry helpers come from the real edit modules
+// on purpose: the probe has to exercise the production conversion, not a copy.
+
+import { offsetCoordinate, polylineLengthMeters, type Coordinate } from '../../src/edit/meters';
+import type { EditHandle } from '../../src/edit/types';
+
+/** The road heads due north, so its normal — the drag axis — points due east. */
+export const TANGENT_AZIMUTH = 0;
+export const AXIS_AZIMUTH = 90;
+export const WIDTH_METERS = 12;
+export const LANE_COUNT = 4;
+
+export const CENTERLINE: Coordinate[] = [
+  [116.397, 39.905],
+  [116.397, 39.915],
+];
+
+export const ROAD_LENGTH_METERS = polylineLengthMeters(CENTERLINE);
+
+const MIDPOINT: Coordinate = [116.397, 39.91];
+
+/** Outer ring of the baseline road surface, EPSG:4326. */
+export function roadSurfaceRing(): Coordinate[] {
+  const half = WIDTH_METERS / 2;
+  const left = CENTERLINE.map((point) => offsetCoordinate(point, TANGENT_AZIMUTH + 90, half));
+  const right = CENTERLINE.map((point) => offsetCoordinate(point, TANGENT_AZIMUTH - 90, half));
+  return [...left, ...right.reverse(), left[0]];
+}
+
+/** Lane divider lines, to make it obvious if the baseline ever gets rewritten. */
+export function laneDividerLines(): Coordinate[][] {
+  const laneWidth = WIDTH_METERS / LANE_COUNT;
+  const lines: Coordinate[][] = [];
+  for (let index = 1; index < LANE_COUNT; index += 1) {
+    const lateral = -WIDTH_METERS / 2 + laneWidth * index;
+    lines.push(CENTERLINE.map((point) => offsetCoordinate(point, TANGENT_AZIMUTH + 90, lateral)));
+  }
+  return lines;
+}
+
+/**
+ * A left edge-offset handle shaped exactly like the one `makeRoadHandles()`
+ * emits, so `projectHandleValue()` sees production input.
+ */
+export const PROBE_HANDLE: EditHandle = {
+  handleId: 'probe:road-edge-offset:left',
+  kind: 'road-edge-offset',
+  anchor: {
+    type: 'road-interval',
+    roadId: 'probe:road/1:forward',
+    startStation: 0.15,
+    endStation: 0.85,
+    side: 'left',
+  },
+  position: offsetCoordinate(MIDPOINT, AXIS_AZIMUTH, WIDTH_METERS / 2),
+  axisAzimuth: AXIS_AZIMUTH,
+  value: { current: 0, min: -WIDTH_METERS * 0.45, max: WIDTH_METERS * 0.45, unit: 'meter' },
+  affects: [],
+  editable: true,
+};