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/Workers.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <functional>
+
+#include "WorkerPool.h"
+
+struct HTTPReply;
+struct HTTPContext;
+
+/**
+ * The dashboard's worker threads (one WorkerPool, scenery_workers threads) for slow work routes would otherwise do
+ * on the web server's one thread: converting the client's models, building a zone's terrain and scene data the
+ * first time it is viewed, converting textures with ImageMagick. What they run must be thread safe: the zone data
+ * builders keep their results in OnceCaches and read the client's files through ClientAssets.
+ */
+namespace Workers {
+	// Start the threads (scenery_workers: 0 picks WorkerPool::DefaultThreads)
+	void Start();
+
+	// Stop them; queued work is dropped (answer deferred requests before Web::Shutdown)
+	void Stop();
+
+	WorkerPool& Pool();
+
+	/**
+	 * Answer a request with what `fill` writes to the reply: right away when `ready` (what it needs is built, so it's
+	 * quick), else from a worker thread (Web::Defer). `fill` runs on either thread, so it must be thread safe and must
+	 * not keep a reference to the request.
+	 */
+	void Reply(HTTPReply& reply, const HTTPContext& context, bool ready, std::function<void(HTTPReply&)> fill,
+		WorkerPool::ePriority priority = WorkerPool::ePriority::NORMAL);
+}