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, 3 insertions, 3 deletions

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,