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, 90 deletions

diff --git a/thirdparty/spdlog/tests/test_stdout_api.cpp b/thirdparty/spdlog/tests/test_stdout_api.cpp
deleted file mode 100644
index 67659b84d..000000000
--- a/thirdparty/spdlog/tests/test_stdout_api.cpp
+++ /dev/null
@@ -1,90 +0,0 @@
-/*
- * This content is released under the MIT License as specified in
- * https://raw.githubusercontent.com/gabime/spdlog/master/LICENSE
- */
-#include "includes.h"
-#include "spdlog/sinks/stdout_sinks.h"
-#include "spdlog/sinks/stdout_color_sinks.h"
-TEST_CASE("stdout_st", "[stdout]") {
-    auto l = spdlog::stdout_logger_st("test");
-    l->set_pattern("%+");
-    l->set_level(spdlog::level::trace);
-    l->trace("Test stdout_st");
-    spdlog::drop_all();
-}
-
-TEST_CASE("stdout_mt", "[stdout]") {
-    auto l = spdlog::stdout_logger_mt("test");
-    l->set_pattern("%+");
-    l->set_level(spdlog::level::debug);
-    l->debug("Test stdout_mt");
-    spdlog::drop_all();
-}
-
-TEST_CASE("stderr_st", "[stderr]") {
-    auto l = spdlog::stderr_logger_st("test");
-    l->set_pattern("%+");
-    l->info("Test stderr_st");
-    spdlog::drop_all();
-}
-
-TEST_CASE("stderr_mt", "[stderr]") {
-    auto l = spdlog::stderr_logger_mt("test");
-    l->set_pattern("%+");
-    l->info("Test stderr_mt");
-    l->warn("Test stderr_mt");
-    l->error("Test stderr_mt");
-    l->critical("Test stderr_mt");
-    spdlog::drop_all();
-}
-
-// color loggers
-TEST_CASE("stdout_color_st", "[stdout]") {
-    auto l = spdlog::stdout_color_st("test");
-    l->set_pattern("%+");
-    l->info("Test stdout_color_st");
-    spdlog::drop_all();
-}
-
-TEST_CASE("stdout_color_mt", "[stdout]") {
-    auto l = spdlog::stdout_color_mt("test");
-    l->set_pattern("%+");
-    l->set_level(spdlog::level::trace);
-    l->trace("Test stdout_color_mt");
-    spdlog::drop_all();
-}
-
-TEST_CASE("stderr_color_st", "[stderr]") {
-    auto l = spdlog::stderr_color_st("test");
-    l->set_pattern("%+");
-    l->set_level(spdlog::level::debug);
-    l->debug("Test stderr_color_st");
-    spdlog::drop_all();
-}
-
-TEST_CASE("stderr_color_mt", "[stderr]") {
-    auto l = spdlog::stderr_color_mt("test");
-    l->set_pattern("%+");
-    l->info("Test stderr_color_mt");
-    l->warn("Test stderr_color_mt");
-    l->error("Test stderr_color_mt");
-    l->critical("Test stderr_color_mt");
-    spdlog::drop_all();
-}
-
-#ifdef SPDLOG_WCHAR_TO_UTF8_SUPPORT
-
-TEST_CASE("wchar_api", "[stdout]") {
-    auto l = spdlog::stdout_logger_st("wchar_logger");
-    l->set_pattern("%+");
-    l->set_level(spdlog::level::trace);
-    l->trace(L"Test wchar_api");
-    l->trace(L"Test wchar_api {}", L"param");
-    l->trace(L"Test wchar_api {}", 1);
-    l->trace(L"Test wchar_api {}", std::wstring{L"wstring param"});
-    l->trace(std::wstring{L"Test wchar_api wstring"});
-    SPDLOG_LOGGER_DEBUG(l, L"Test SPDLOG_LOGGER_DEBUG {}", L"param");
-    spdlog::drop_all();
-}
-
-#endif