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

1 files changed, 88 insertions, 0 deletions

diff --git a/src/zencore/logging/tracesink.cpp b/src/zencore/logging/tracesink.cpp
new file mode 100644
index 000000000..e3533327b
--- /dev/null
+++ b/src/zencore/logging/tracesink.cpp
@@ -0,0 +1,88 @@
+
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#include <zencore/logbase.h>
+#include <zencore/logging/tracesink.h>
+#include <zencore/string.h>
+#include <zencore/timer.h>
+#include <zencore/trace.h>
+
+namespace zen::logging {
+
+UE_TRACE_CHANNEL_DEFINE(LogChannel)
+
+UE_TRACE_EVENT_BEGIN(Logging, LogCategory, NoSync | Important)
+	UE_TRACE_EVENT_FIELD(const void*, CategoryPointer)
+	UE_TRACE_EVENT_FIELD(uint8_t, DefaultVerbosity)
+	UE_TRACE_EVENT_FIELD(UE::Trace::AnsiString, Name)
+UE_TRACE_EVENT_END()
+
+UE_TRACE_EVENT_BEGIN(Logging, LogMessageSpec, NoSync | Important)
+	UE_TRACE_EVENT_FIELD(const void*, LogPoint)
+	UE_TRACE_EVENT_FIELD(const void*, CategoryPointer)
+	UE_TRACE_EVENT_FIELD(int32_t, Line)
+	UE_TRACE_EVENT_FIELD(uint8_t, Verbosity)
+	UE_TRACE_EVENT_FIELD(UE::Trace::AnsiString, FileName)
+	UE_TRACE_EVENT_FIELD(UE::Trace::AnsiString, FormatString)
+UE_TRACE_EVENT_END()
+
+UE_TRACE_EVENT_BEGIN(Logging, LogMessage, NoSync)
+	UE_TRACE_EVENT_FIELD(const void*, LogPoint)
+	UE_TRACE_EVENT_FIELD(uint64_t, Cycle)
+	UE_TRACE_EVENT_FIELD(uint8_t[], FormatArgs)
+UE_TRACE_EVENT_END()
+
+void
+TraceLogCategory(const logging::Logger* Category, const char* Name, logging::LogLevel DefaultVerbosity)
+{
+	uint16_t NameLen = uint16_t(strlen(Name));
+	UE_TRACE_LOG(Logging, LogCategory, LogChannel, NameLen * sizeof(ANSICHAR))
+		<< LogCategory.CategoryPointer(Category) << LogCategory.DefaultVerbosity(uint8_t(DefaultVerbosity))
+		<< LogCategory.Name(Name, NameLen);
+}
+
+void
+TraceLogMessageSpec(const void*			   LogPoint,
+					const logging::Logger* Category,
+					logging::LogLevel	   Verbosity,
+					const std::string_view File,
+					int32_t				   Line,
+					const std::string_view Format)
+{
+	uint16_t FileNameLen	 = uint16_t(File.size());
+	uint16_t FormatStringLen = uint16_t(Format.size());
+	uint32_t DataSize		 = (FileNameLen * sizeof(ANSICHAR)) + (FormatStringLen * sizeof(ANSICHAR));
+	UE_TRACE_LOG(Logging, LogMessageSpec, LogChannel, DataSize)
+		<< LogMessageSpec.LogPoint(LogPoint) << LogMessageSpec.CategoryPointer(Category) << LogMessageSpec.Line(Line)
+		<< LogMessageSpec.Verbosity(uint8_t(Verbosity)) << LogMessageSpec.FileName(File.data(), FileNameLen)
+		<< LogMessageSpec.FormatString(Format.data(), FormatStringLen);
+}
+
+void
+TraceLogMessageInternal(const void* LogPoint, int32_t EncodedFormatArgsSize, const uint8_t* EncodedFormatArgs)
+{
+	UE_TRACE_LOG(Logging, LogMessage, LogChannel) << LogMessage.LogPoint(LogPoint) << LogMessage.Cycle(GetHifreqTimerValue())
+												  << LogMessage.FormatArgs(EncodedFormatArgs, EncodedFormatArgsSize);
+}
+
+//////////////////////////////////////////////////////////////////////////
+
+void
+TraceSink::Log(const LogMessage& Msg)
+{
+	ZEN_UNUSED(Msg);
+}
+
+void
+TraceSink::Flush()
+{
+}
+
+void
+TraceSink::SetFormatter(std::unique_ptr<Formatter> /*InFormatter*/)
+{
+	// This sink doesn't use a formatter since it just forwards the raw format
+	// args to the trace system
+}
+
+}  // namespace zen::logging