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, 14 deletions

diff --git a/thirdparty/spdlog/tests/test_systemd.cpp b/thirdparty/spdlog/tests/test_systemd.cpp
deleted file mode 100644
index e36365741..000000000
--- a/thirdparty/spdlog/tests/test_systemd.cpp
+++ /dev/null
@@ -1,14 +0,0 @@
-#include "includes.h"
-#include "spdlog/sinks/systemd_sink.h"
-
-TEST_CASE("systemd", "[all]") {
-    auto systemd_sink = std::make_shared<spdlog::sinks::systemd_sink_st>();
-    spdlog::logger logger("spdlog_systemd_test", systemd_sink);
-    logger.set_level(spdlog::level::trace);
-    logger.trace("test spdlog trace");
-    logger.debug("test spdlog debug");
-    SPDLOG_LOGGER_INFO((&logger), "test spdlog info");
-    SPDLOG_LOGGER_WARN((&logger), "test spdlog warn");
-    SPDLOG_LOGGER_ERROR((&logger), "test spdlog error");
-    SPDLOG_LOGGER_CRITICAL((&logger), "test spdlog critical");
-}