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, 5 insertions, 3 deletions

diff --git a/src/zenserver-test/zenserver-test.cpp b/src/zenserver-test/zenserver-test.cpp
index bd36d731f..8d5400294 100644
--- a/src/zenserver-test/zenserver-test.cpp
+++ b/src/zenserver-test/zenserver-test.cpp
@@ -9,6 +9,7 @@
 #	include <zencore/except.h>
 #	include <zencore/fmtutils.h>
 #	include <zencore/logging.h>
+#	include <zencore/logging/registry.h>
 #	include <zencore/stream.h>
 #	include <zencore/string.h>
 #	include <zencore/testutils.h>
@@ -17,7 +18,7 @@
 #	include <zenhttp/httpclient.h>
 #	include <zenhttp/packageformat.h>
 #	include <zenutil/config/commandlineoptions.h>
-#	include <zenutil/logging/testformatter.h>
+#	include <zenutil/logging/fullformatter.h>
 #	include <zenutil/zenserverprocess.h>
 
 #	include <atomic>
@@ -85,8 +86,9 @@ main(int argc, char** argv)
 
 	zen::logging::InitializeLogging();
 
-	zen::logging::SetLogLevel(zen::logging::level::Debug);
-	spdlog::set_formatter(std::make_unique<zen::logging::full_test_formatter>("test", std::chrono::system_clock::now()));
+	zen::logging::SetLogLevel(zen::logging::Debug);
+	zen::logging::Registry::Instance().SetFormatter(
+		std::make_unique<zen::logging::FullFormatter>("test", std::chrono::system_clock::now()));
 
 	std::filesystem::path ProgramBaseDir = GetRunningExecutablePath().parent_path();
 	std::filesystem::path TestBaseDir	 = std::filesystem::current_path() / ".test";