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

6 files changed, 38 insertions, 46 deletions

diff --git a/src/zenserver/diag/diagsvcs.cpp b/src/zenserver/diag/diagsvcs.cpp
index d8d53b0e3..5fa81ff9f 100644
--- a/src/zenserver/diag/diagsvcs.cpp
+++ b/src/zenserver/diag/diagsvcs.cpp
@@ -12,9 +12,7 @@
 #include <fstream>
 #include <sstream>
 
-ZEN_THIRD_PARTY_INCLUDES_START
-#include <spdlog/logger.h>
-ZEN_THIRD_PARTY_INCLUDES_END
+#include <zencore/logging/logger.h>
 
 namespace zen {
 
@@ -64,7 +62,7 @@ HttpHealthService::HttpHealthService()
 		[this](HttpRouterRequest& RoutedReq) {
 			HttpServerRequest& HttpReq = RoutedReq.ServerRequest();
 
-			zen::Log().SpdLogger->flush();
+			zen::Log().Flush();
 
 			std::filesystem::path Path = [&] {
 				RwLock::SharedLockScope _(m_InfoLock);
diff --git a/src/zenserver/diag/logging.cpp b/src/zenserver/diag/logging.cpp
index 75a8efc09..178c3d3b5 100644
--- a/src/zenserver/diag/logging.cpp
+++ b/src/zenserver/diag/logging.cpp
@@ -6,6 +6,8 @@
 
 #include <zencore/filesystem.h>
 #include <zencore/fmtutils.h>
+#include <zencore/logging/logger.h>
+#include <zencore/logging/registry.h>
 #include <zencore/memory/llm.h>
 #include <zencore/session.h>
 #include <zencore/string.h>
@@ -14,10 +16,6 @@
 
 #include "otlphttp.h"
 
-ZEN_THIRD_PARTY_INCLUDES_START
-#include <spdlog/spdlog.h>
-ZEN_THIRD_PARTY_INCLUDES_END
-
 namespace zen {
 
 void
@@ -43,13 +41,12 @@ InitializeServerLogging(const ZenServerConfig& InOptions, bool WithCacheService)
 	std::filesystem::path HttpLogPath = InOptions.DataDir / "logs" / "http.log";
 	zen::CreateDirectories(HttpLogPath.parent_path());
 
-	auto HttpSink	= std::make_shared<zen::logging::RotatingFileSink>(HttpLogPath,
-																	   /* max size */ 128 * 1024 * 1024,
-																	   /* max files */ 16,
-																	   /* rotate on open */ true);
-	auto HttpLogger = std::make_shared<spdlog::logger>("http_requests", HttpSink);
-	spdlog::apply_logger_env_levels(HttpLogger);
-	spdlog::register_logger(HttpLogger);
+	logging::SinkPtr	 HttpSink(new zen::logging::RotatingFileSink(HttpLogPath,
+																 /* max size */ 128 * 1024 * 1024,
+																 /* max files */ 16,
+																 /* rotate on open */ true));
+	Ref<logging::Logger> HttpLogger(new logging::Logger("http_requests", std::vector<logging::SinkPtr>{HttpSink}));
+	logging::Registry::Instance().Register(HttpLogger);
 
 	if (WithCacheService)
 	{
@@ -57,33 +54,30 @@ InitializeServerLogging(const ZenServerConfig& InOptions, bool WithCacheService)
 		std::filesystem::path CacheLogPath = InOptions.DataDir / "logs" / "z$.log";
 		zen::CreateDirectories(CacheLogPath.parent_path());
 
-		auto CacheSink	 = std::make_shared<zen::logging::RotatingFileSink>(CacheLogPath,
-																			/* max size */ 128 * 1024 * 1024,
-																			/* max files */ 16,
-																			/* rotate on open */ false);
-		auto CacheLogger = std::make_shared<spdlog::logger>("z$", CacheSink);
-		spdlog::apply_logger_env_levels(CacheLogger);
-		spdlog::register_logger(CacheLogger);
+		logging::SinkPtr	 CacheSink(new zen::logging::RotatingFileSink(CacheLogPath,
+																	  /* max size */ 128 * 1024 * 1024,
+																	  /* max files */ 16,
+																	  /* rotate on open */ false));
+		Ref<logging::Logger> CacheLogger(new logging::Logger("z$", std::vector<logging::SinkPtr>{CacheSink}));
+		logging::Registry::Instance().Register(CacheLogger);
 
 		// Jupiter - only log upstream HTTP traffic to file
 
-		auto JupiterLogger = std::make_shared<spdlog::logger>("jupiter", FileSink);
-		spdlog::apply_logger_env_levels(JupiterLogger);
-		spdlog::register_logger(JupiterLogger);
+		Ref<logging::Logger> JupiterLogger(new logging::Logger("jupiter", std::vector<logging::SinkPtr>{FileSink}));
+		logging::Registry::Instance().Register(JupiterLogger);
 
 		// Zen - only log upstream HTTP traffic to file
 
-		auto ZenClientLogger = std::make_shared<spdlog::logger>("zenclient", FileSink);
-		spdlog::apply_logger_env_levels(ZenClientLogger);
-		spdlog::register_logger(ZenClientLogger);
+		Ref<logging::Logger> ZenClientLogger(new logging::Logger("zenclient", std::vector<logging::SinkPtr>{FileSink}));
+		logging::Registry::Instance().Register(ZenClientLogger);
 	}
 
 #if ZEN_WITH_OTEL
 	if (!InOptions.LoggingConfig.OtelEndpointUri.empty())
 	{
 		// TODO: Should sanity check that endpoint is reachable? Also, a valid URI?
-		auto OtelSink = std::make_shared<zen::logging::OtelHttpProtobufSink>(InOptions.LoggingConfig.OtelEndpointUri);
-		zen::logging::Default().SpdLogger->sinks().push_back(std::move(OtelSink));
+		logging::SinkPtr OtelSink(new zen::logging::OtelHttpProtobufSink(InOptions.LoggingConfig.OtelEndpointUri));
+		zen::logging::Default()->AddSink(std::move(OtelSink));
 	}
 #endif
 
@@ -91,9 +85,10 @@ InitializeServerLogging(const ZenServerConfig& InOptions, bool WithCacheService)
 
 	const zen::Oid ServerSessionId = zen::GetSessionId();
 
-	spdlog::apply_all([&](auto Logger) {
+	static constinit logging::LogPoint SessionIdPoint{{}, logging::Info, "server session id: {}"};
+	logging::Registry::Instance().ApplyAll([&](auto Logger) {
 		ZEN_MEMSCOPE(ELLMTag::Logging);
-		Logger->info("server session id: {}", ServerSessionId);
+		Logger->Log(SessionIdPoint, fmt::make_format_args(ServerSessionId));
 	});
 }
 
diff --git a/src/zenserver/diag/otlphttp.cpp b/src/zenserver/diag/otlphttp.cpp
index d62ccccb6..1434c9331 100644
--- a/src/zenserver/diag/otlphttp.cpp
+++ b/src/zenserver/diag/otlphttp.cpp
@@ -53,7 +53,7 @@ OtelHttpProtobufSink::TraceRecorder::RecordSpans(zen::otel::TraceId Trace, std::
 }
 
 void
-OtelHttpProtobufSink::log(const spdlog::details::log_msg& Msg)
+OtelHttpProtobufSink::Log(const LogMessage& Msg)
 {
 	{
 		std::string Data = m_Encoder.FormatOtelProtobuf(Msg);
@@ -74,7 +74,7 @@ OtelHttpProtobufSink::log(const spdlog::details::log_msg& Msg)
 	}
 }
 void
-OtelHttpProtobufSink::flush()
+OtelHttpProtobufSink::Flush()
 {
 }
 
diff --git a/src/zenserver/diag/otlphttp.h b/src/zenserver/diag/otlphttp.h
index 2281bdcc0..8254af04d 100644
--- a/src/zenserver/diag/otlphttp.h
+++ b/src/zenserver/diag/otlphttp.h
@@ -3,7 +3,7 @@
 
 #pragma once
 
-#include <spdlog/sinks/sink.h>
+#include <zencore/logging/sink.h>
 #include <zencore/zencore.h>
 #include <zenhttp/httpclient.h>
 #include <zentelemetry/otlpencoder.h>
@@ -14,12 +14,12 @@
 namespace zen::logging {
 
 /**
- * OTLP/HTTP sink for spdlog
+ * OTLP/HTTP sink for logging
  *
  * Sends log messages and traces to an OpenTelemetry collector via OTLP over HTTP
  */
 
-class OtelHttpProtobufSink : public spdlog::sinks::sink
+class OtelHttpProtobufSink : public Sink
 {
 public:
 	// Note that this URI should be the base URI of the OTLP HTTP endpoint, e.g.
@@ -31,10 +31,9 @@ public:
 	OtelHttpProtobufSink& operator=(const OtelHttpProtobufSink&) = delete;
 
 private:
-	virtual void log(const spdlog::details::log_msg& Msg) override;
-	virtual void flush() override;
-	virtual void set_pattern(const std::string& pattern) override { ZEN_UNUSED(pattern); }
-	virtual void set_formatter(std::unique_ptr<spdlog::formatter> sink_formatter) override { ZEN_UNUSED(sink_formatter); }
+	virtual void Log(const LogMessage& Msg) override;
+	virtual void Flush() override;
+	virtual void SetFormatter(std::unique_ptr<Formatter>) override {}
 
 	void RecordSpans(zen::otel::TraceId Trace, std::span<const zen::otel::Span*> Spans);
 
@@ -61,4 +60,4 @@ private:
 
 }  // namespace zen::logging
 
-#endif
\ No newline at end of file
+#endif
diff --git a/src/zenserver/main.cpp b/src/zenserver/main.cpp
index c764cbde6..09ecc48e5 100644
--- a/src/zenserver/main.cpp
+++ b/src/zenserver/main.cpp
@@ -246,7 +246,7 @@ test_main(int argc, char** argv)
 #	endif	// ZEN_PLATFORM_WINDOWS
 
 	zen::logging::InitializeLogging();
-	zen::logging::SetLogLevel(zen::logging::level::Debug);
+	zen::logging::SetLogLevel(zen::logging::Debug);
 
 	zen::MaximizeOpenFileCount();
 
diff --git a/src/zenserver/storage/admin/admin.cpp b/src/zenserver/storage/admin/admin.cpp
index 19155e02b..c9f999c69 100644
--- a/src/zenserver/storage/admin/admin.cpp
+++ b/src/zenserver/storage/admin/admin.cpp
@@ -716,7 +716,7 @@ HttpAdminService::HttpAdminService(GcScheduler&			   Scheduler,
 		"logs",
 		[this](HttpRouterRequest& Req) {
 			CbObjectWriter Obj;
-			auto		   LogLevel = logging::level::ToStringView(logging::GetLogLevel());
+			auto		   LogLevel = logging::ToStringView(logging::GetLogLevel());
 			Obj.AddString("loglevel", std::string_view(LogLevel.data(), LogLevel.size()));
 			Obj.AddString("Logfile", PathToUtf8(m_LogPaths.AbsLogPath));
 			Obj.BeginObject("cache");
@@ -767,8 +767,8 @@ HttpAdminService::HttpAdminService(GcScheduler&			   Scheduler,
 			}
 			if (std::string Param(Params.GetValue("loglevel")); Param.empty() == false)
 			{
-				logging::level::LogLevel NewLevel = logging::level::ParseLogLevelString(Param);
-				std::string_view		 LogLevel = logging::level::ToStringView(NewLevel);
+				logging::LogLevel NewLevel = logging::ParseLogLevelString(Param);
+				std::string_view  LogLevel = logging::ToStringView(NewLevel);
 				if (LogLevel != Param)
 				{
 					return Req.ServerRequest().WriteResponse(HttpResponseCode::BadRequest,