feat: build a zone's 3D data on worker threads

Opening a zone the first time built its terrain, scene objects and manifests
and ran ImageMagick on the web thread, stalling the dashboard for seconds.

- Workers: the shared pool plus Workers::Reply (answer at once when built,
  else from a worker via Web::Defer).
- terrain_chunks/terrain_layers/scene/paths/scenery/flairs (world3d, property
  and showcase routes) and terrain textures go through it; results are built
  once in OnceCaches, the .raw is read once per zone for chunks, layers and
  flairs, deflated bodies are cached thread-safely.
- ImageMagick conversions are deduplicated and written under a temporary name.
- Workers don't query the CDClient, read settings or call mongoose: ZoneTable,
  render components, flairs, object names, LOT kinds and terrain texture names
  are read at startup; client_location is read once; base64 is plain C++.
- Logger writes one line at a time (mutex; localtime's buffer is shared).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dDashboardServer/routes/WorldView.h

@@ -10,6 +10,9 @@ struct PlayerPositions;
 namespace WorldView {
 	void RegisterRoutes();
 
+	// Read the CDClient data the zone data builders need (at startup: they run on worker threads)
+	void Preload();
+
 	// A world server reported its players' positions: keep some of them for replays (throttled, written in batches)
 	void RecordPositions(const PlayerPositions& positions);
 }