Eliminate spdlog dependency (#773)

Removes the vendored spdlog library (~12,000 lines) and replaces it with a purpose-built logging system in zencore (~1,800 lines). The new implementation provides the same functionality with fewer abstractions, no shared_ptr overhead, and full control over the logging pipeline.

### What changed

**New logging core in zencore/logging/:**
- LogMessage, Formatter, Sink, Logger, Registry - core abstractions matching spdlog's model but simplified
- AnsiColorStdoutSink - ANSI color console output (replaces spdlog stdout_color_sink)
- MsvcSink - OutputDebugString on Windows (replaces spdlog msvc_sink)
- AsyncSink - async logging via BlockingQueue worker thread (replaces spdlog async_logger)
- NullSink, MessageOnlyFormatter - utility types
- Thread-safe timestamp caching in formatters using RwLock

**Moved to zenutil/logging/:**
- FullFormatter - full log formatting with timestamp, logger name, level, source location, multiline alignment
- JsonFormatter - structured JSON log output
- RotatingFileSink - rotating file sink with atomic size tracking

**API changes:**
- Log levels are now an enum (LogLevel) instead of int, eliminating the zen::logging::level namespace
- LoggerRef no longer wraps shared_ptr - it holds a raw pointer with the registry owning lifetime
- Logger error handler is wired through Registry and propagated to all loggers on registration
- Logger::Log() now populates ThreadId on every message

**Cleanup:**
- Deleted thirdparty/spdlog/ entirely (110+ files)
- Deleted full_test_formatter (was ~80% duplicate of FullFormatter)
- Renamed snake_case classes to PascalCase (full_formatter -> FullFormatter, json_formatter -> JsonFormatter, sentry_sink -> SentrySink)
- Removed spdlog from xmake dependency graph

### Build / test impact

- zencore no longer depends on spdlog
- zenutil and zenvfs xmake.lua updated to drop spdlog dep
- zentelemetry xmake.lua updated to drop spdlog dep
- All existing tests pass, no test changes required beyond formatter class renames

3 files changed, 31 insertions, 15 deletions

diff --git a/src/zenhttp/servers/httpasio.cpp b/src/zenhttp/servers/httpasio.cpp
index 33f182df9..2cf051d14 100644
--- a/src/zenhttp/servers/httpasio.cpp
+++ b/src/zenhttp/servers/httpasio.cpp
@@ -150,7 +150,7 @@ inline LoggerRef
 InitLogger()
 {
 	LoggerRef Logger = logging::Get("asio");
-	// Logger.SetLogLevel(logging::level::Trace);
+	// Logger.SetLogLevel(logging::Trace);
 	return Logger;
 }
 
@@ -1256,7 +1256,7 @@ HttpServerConnection::HandleRequest()
 		const HttpVerb		   RequestVerb = Request.RequestVerb();
 		const std::string_view Uri		   = Request.RelativeUri();
 
-		if (m_Server.m_RequestLog.ShouldLog(logging::level::Trace))
+		if (m_Server.m_RequestLog.ShouldLog(logging::Trace))
 		{
 			ZEN_LOG_TRACE(m_Server.m_RequestLog,
 						  "connection #{} Handling Request: {} {} ({} bytes ({}), accept: {})",
diff --git a/src/zenhttp/servers/httpplugin.cpp b/src/zenhttp/servers/httpplugin.cpp
index 850dafdca..021b941bd 100644
--- a/src/zenhttp/servers/httpplugin.cpp
+++ b/src/zenhttp/servers/httpplugin.cpp
@@ -383,7 +383,7 @@ HttpPluginConnectionHandler::HandleRequest()
 		const HttpVerb		   RequestVerb = Request.RequestVerb();
 		const std::string_view Uri		   = Request.RelativeUri();
 
-		if (m_Server->m_RequestLog.ShouldLog(logging::level::Trace))
+		if (m_Server->m_RequestLog.ShouldLog(logging::Trace))
 		{
 			ZEN_LOG_TRACE(m_Server->m_RequestLog,
 						  "connection #{} Handling Request: {} {} ({} bytes ({}), accept: {})",
@@ -480,7 +480,7 @@ HttpPluginConnectionHandler::HandleRequest()
 
 			const std::vector<IoBuffer>& ResponseBuffers = Response->ResponseBuffers();
 
-			if (m_Server->m_RequestLog.ShouldLog(logging::level::Trace))
+			if (m_Server->m_RequestLog.ShouldLog(logging::Trace))
 			{
 				m_Server->m_RequestTracer.WriteDebugPayload(fmt::format("response_{}_{}.bin", m_ConnectionId, RequestNumber),
 															ResponseBuffers);
diff --git a/src/zenhttp/transports/dlltransport.cpp b/src/zenhttp/transports/dlltransport.cpp
index 9135d5425..489324aba 100644
--- a/src/zenhttp/transports/dlltransport.cpp
+++ b/src/zenhttp/transports/dlltransport.cpp
@@ -72,20 +72,36 @@ DllTransportLogger::DllTransportLogger(std::string_view PluginName) : m_PluginNa
 void
 DllTransportLogger::LogMessage(LogLevel PluginLogLevel, const char* Message)
 {
-	logging::level::LogLevel Level;
-	// clang-format off
 	switch (PluginLogLevel)
 	{
-		case LogLevel::Trace:    Level = logging::level::Trace;    break;
-		case LogLevel::Debug:    Level = logging::level::Debug;    break;
-		case LogLevel::Info:     Level = logging::level::Info;     break;
-		case LogLevel::Warn:     Level = logging::level::Warn;     break;
-		case LogLevel::Err:      Level = logging::level::Err;      break;
-		case LogLevel::Critical: Level = logging::level::Critical; break;
-		default:                 Level = logging::level::Off;      break;
+		case LogLevel::Trace:
+			ZEN_TRACE("[{}] {}", m_PluginName, Message);
+			return;
+
+		case LogLevel::Debug:
+			ZEN_DEBUG("[{}] {}", m_PluginName, Message);
+			return;
+
+		case LogLevel::Info:
+			ZEN_INFO("[{}] {}", m_PluginName, Message);
+			return;
+
+		case LogLevel::Warn:
+			ZEN_WARN("[{}] {}", m_PluginName, Message);
+			return;
+
+		case LogLevel::Err:
+			ZEN_ERROR("[{}] {}", m_PluginName, Message);
+			return;
+
+		case LogLevel::Critical:
+			ZEN_CRITICAL("[{}] {}", m_PluginName, Message);
+			return;
+
+		default:
+			ZEN_UNUSED(Message);
+			break;
 	}
-	// clang-format on
-	ZEN_LOG(Log(), Level, "[{}] {}", m_PluginName, Message)
 }
 
 uint32_t