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>

dCommon/Logger.cpp

@@ -3,8 +3,15 @@
 #include <algorithm>
 #include <ctime>
 #include <filesystem>
+#include <mutex>
 #include <stdarg.h>
 
+namespace {
+	// Servers with worker threads (the dashboard) log from several threads: one line is written at a time, and
+	// localtime's buffer is shared
+	std::mutex g_LogMutex;
+}
+
 Writer::~Writer() {
 	// Flush before we close
 	Flush();
@@ -49,6 +56,7 @@ Logger::Logger(const std::string& outpath, bool logToConsole, bool logDebugState
 }
 
 void Logger::vLog(const char* format, va_list args) {
+	std::lock_guard lock(g_LogMutex);
 	time_t t = time(NULL);
 	struct tm* time = localtime(&t);
 	char timeStr[70];
@@ -78,6 +86,7 @@ void Logger::LogDebug(const char* className, const char* format, ...) {
 }
 
 void Logger::Flush() {
+	std::lock_guard lock(g_LogMutex);
 	for (const auto& writer : m_Writers) {
 		writer->Flush();
 	}