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, 0 insertions, 42 deletions

diff --git a/thirdparty/spdlog/tests/includes.h b/thirdparty/spdlog/tests/includes.h
deleted file mode 100644
index 2e49a5cb7..000000000
--- a/thirdparty/spdlog/tests/includes.h
+++ /dev/null
@@ -1,42 +0,0 @@
-#pragma once
-
-#if defined(__GNUC__) && __GNUC__ == 12
-    #pragma GCC diagnostic push
-    #pragma GCC diagnostic ignored "-Wmaybe-uninitialized"  // Workaround for GCC 12
-#endif
-#include <catch2/catch_all.hpp>
-#if defined(__GNUC__) && __GNUC__ == 12
-    #pragma GCC diagnostic pop
-#endif
-
-#include "utils.h"
-#include <chrono>
-#include <cstdio>
-#include <exception>
-#include <fstream>
-#include <iostream>
-#include <ostream>
-#include <sstream>
-#include <string>
-#include <iomanip>
-#include <stdlib.h>
-
-#define SPDLOG_ACTIVE_LEVEL SPDLOG_LEVEL_DEBUG
-
-#include "spdlog/spdlog.h"
-#include "spdlog/async.h"
-#include "spdlog/details/fmt_helper.h"
-#include "spdlog/details/os.h"
-
-#ifndef SPDLOG_NO_TLS
-    #include "spdlog/mdc.h"
-#endif
-
-#include "spdlog/sinks/basic_file_sink.h"
-#include "spdlog/sinks/daily_file_sink.h"
-#include "spdlog/sinks/null_sink.h"
-#include "spdlog/sinks/ostream_sink.h"
-#include "spdlog/sinks/rotating_file_sink.h"
-#include "spdlog/sinks/stdout_color_sinks.h"
-#include "spdlog/sinks/msvc_sink.h"
-#include "spdlog/pattern_formatter.h"