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, 4 insertions, 4 deletions

diff --git a/src/zentelemetry/include/zentelemetry/otlpencoder.h b/src/zentelemetry/include/zentelemetry/otlpencoder.h
index ed6665781..f280aa9ec 100644
--- a/src/zentelemetry/include/zentelemetry/otlpencoder.h
+++ b/src/zentelemetry/include/zentelemetry/otlpencoder.h
@@ -13,9 +13,9 @@
 #	include <protozero/pbf_builder.hpp>
 #	include <protozero/types.hpp>
 
-namespace spdlog { namespace details {
-	struct log_msg;
-}}	// namespace spdlog::details
+namespace zen::logging {
+struct LogMessage;
+}  // namespace zen::logging
 
 namespace zen::otel {
 enum class Resource : protozero::pbf_tag_type;
@@ -46,7 +46,7 @@ public:
 	void AddResourceAttribute(const std::string_view& Key, const std::string_view& Value);
 	void AddResourceAttribute(const std::string_view& Key, int64_t Value);
 
-	std::string FormatOtelProtobuf(const spdlog::details::log_msg& Msg) const;
+	std::string FormatOtelProtobuf(const logging::LogMessage& Msg) const;
 	std::string FormatOtelMetrics() const;
 	std::string FormatOtelTrace(zen::otel::TraceId Trace, std::span<const zen::otel::Span*> Spans) const;