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/probe.css

@@ -0,0 +1,148 @@
+/* Probe-only styles. Deliberately not sharing the workbench stylesheet, so the
+   whole probe can be deleted in one directory. */
+
+* {
+  box-sizing: border-box;
+}
+body {
+  margin: 0;
+  font:
+    14px/1.5 system-ui,
+    -apple-system,
+    'Segoe UI',
+    sans-serif;
+  color: #1b2426;
+  background: #f2f4f3;
+}
+.probe-nav {
+  display: flex;
+  gap: 16px;
+  align-items: center;
+  padding: 10px 16px;
+  background: #1b2426;
+  color: #f2f4f3;
+}
+.probe-nav span {
+  color: #9db0b3;
+  font-size: 13px;
+}
+.probe-nav button {
+  margin-left: auto;
+}
+button {
+  padding: 6px 12px;
+  border: 1px solid #4a5a5d;
+  border-radius: 6px;
+  background: #f2f4f3;
+  cursor: pointer;
+  font: inherit;
+}
+.probe {
+  display: grid;
+  grid-template-columns: 1fr 340px;
+  height: calc(100vh - 45px);
+}
+.probe-map {
+  height: 100%;
+  background: #dfe4e2;
+}
+.probe-panel {
+  padding: 16px;
+  overflow-y: auto;
+  background: #fff;
+  border-left: 1px solid #d3dad8;
+}
+.probe-panel h1 {
+  margin: 0 0 4px;
+  font-size: 16px;
+}
+.probe-panel p {
+  margin: 0 0 12px;
+  color: #59696c;
+}
+.probe-panel dt {
+  margin-top: 12px;
+  font-weight: 600;
+}
+.probe-panel dd {
+  margin: 4px 0 0;
+  color: #35474a;
+}
+.probe-panel ul {
+  margin: 16px 0 0;
+  padding-left: 18px;
+  color: #59696c;
+  font-variant-numeric: tabular-nums;
+}
+.probe-panel h2 {
+  margin: 0 0 6px;
+  font-size: 13px;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: #59696c;
+}
+.probe-panel section {
+  margin-bottom: 20px;
+}
+.probe-panel ol {
+  margin: 0 0 8px;
+  padding-left: 20px;
+}
+.probe-panel ol li {
+  margin-bottom: 4px;
+}
+.probe-howto {
+  padding: 12px;
+  background: #eef4f2;
+  border-left: 3px solid #296956;
+  border-radius: 4px;
+}
+.probe-howto p,
+.probe-ask p {
+  margin: 8px 0 0;
+  font-size: 13px;
+}
+.probe-live {
+  padding: 12px;
+  background: #1b2426;
+  border-radius: 6px;
+}
+.probe-live h2,
+.probe-live p {
+  color: #9db0b3;
+}
+.probe-big {
+  display: block;
+  margin: 2px 0 6px;
+  font-size: 30px;
+  font-variant-numeric: tabular-nums;
+  color: #4ad2f0;
+}
+.probe-ask {
+  padding: 12px;
+  background: #fdf6e8;
+  border-left: 3px solid #d49318;
+  border-radius: 4px;
+}
+.probe-empty {
+  padding: 24px;
+}
+.probe-error {
+  padding: 24px;
+  max-width: 900px;
+}
+.probe-error h1 {
+  margin: 0 0 4px;
+  font-size: 18px;
+  color: #bf3b2e;
+}
+.probe-error pre {
+  padding: 12px;
+  overflow-x: auto;
+  background: #1b2426;
+  color: #f2f4f3;
+  border-radius: 6px;
+  font-size: 12px;
+  white-space: pre-wrap;
+  word-break: break-word;
+}