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/main.tsx

@@ -0,0 +1,61 @@
+import { Component, StrictMode, useState, type ReactNode } from 'react';
+import { createRoot } from 'react-dom/client';
+import { ProbeMap } from './ProbeMap';
+import './probe.css';
+
+// A probe that white-screens teaches nothing: the run has to say *whose* bug it
+// was, ol-ext's or ours. This boundary keeps the error on the page so the gate
+// result stays attributable.
+class ErrorBoundary extends Component<{ children: ReactNode }, { error: Error | null }> {
+  state: { error: Error | null } = { error: null };
+
+  static getDerivedStateFromError(error: Error) {
+    return { error };
+  }
+
+  render() {
+    const { error } = this.state;
+    if (!error) return this.props.children;
+    return (
+      <section className="probe-error">
+        <h1>探针崩溃了</h1>
+        <p>请把下面整段发给我 —— 它决定这次失败算谁的。</p>
+        <pre>
+          {error.name}: {error.message}
+          {'\n\n'}
+          {error.stack}
+        </pre>
+        <button type="button" onClick={() => this.setState({ error: null })}>
+          重试
+        </button>
+      </section>
+    );
+  }
+}
+
+// The mount toggle exists so the React lifecycle question gets answered too: a
+// remount is *expected* to build a new Map, while dragging and re-rendering must
+// not. Gate 1 is about the latter.
+function Probe() {
+  const [mounted, setMounted] = useState(true);
+  return (
+    <>
+      <nav className="probe-nav">
+        <strong>ol-ext Transform 限时探针</strong>
+        <span>隔离页面，不接 API，不影响生产图层</span>
+        <button type="button" onClick={() => setMounted(!mounted)}>
+          {mounted ? '卸载地图' : '重新挂载地图'}
+        </button>
+      </nav>
+      {mounted ? <ProbeMap /> : <p className="probe-empty">地图已卸载。重新挂载会构造新的 Map，这是预期行为。</p>}
+    </>
+  );
+}
+
+createRoot(document.getElementById('probe')!).render(
+  <StrictMode>
+    <ErrorBoundary>
+      <Probe />
+    </ErrorBoundary>
+  </StrictMode>,
+);