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, 73 deletions

diff --git a/thirdparty/spdlog/tests/test_backtrace.cpp b/thirdparty/spdlog/tests/test_backtrace.cpp
deleted file mode 100644
index 4d78c0c21..000000000
--- a/thirdparty/spdlog/tests/test_backtrace.cpp
+++ /dev/null
@@ -1,73 +0,0 @@
-#include "includes.h"
-#include "test_sink.h"
-#include "spdlog/async.h"
-
-TEST_CASE("bactrace1", "[bactrace]") {
-    using spdlog::sinks::test_sink_st;
-    auto test_sink = std::make_shared<test_sink_st>();
-    size_t backtrace_size = 5;
-
-    spdlog::logger logger("test-backtrace", test_sink);
-    logger.set_pattern("%v");
-    logger.enable_backtrace(backtrace_size);
-
-    logger.info("info message");
-    for (int i = 0; i < 100; i++) logger.debug("debug message {}", i);
-
-    REQUIRE(test_sink->lines().size() == 1);
-    REQUIRE(test_sink->lines()[0] == "info message");
-
-    logger.dump_backtrace();
-    REQUIRE(test_sink->lines().size() == backtrace_size + 3);
-    REQUIRE(test_sink->lines()[1] == "****************** Backtrace Start ******************");
-    REQUIRE(test_sink->lines()[2] == "debug message 95");
-    REQUIRE(test_sink->lines()[3] == "debug message 96");
-    REQUIRE(test_sink->lines()[4] == "debug message 97");
-    REQUIRE(test_sink->lines()[5] == "debug message 98");
-    REQUIRE(test_sink->lines()[6] == "debug message 99");
-    REQUIRE(test_sink->lines()[7] == "****************** Backtrace End ********************");
-}
-
-TEST_CASE("bactrace-empty", "[bactrace]") {
-    using spdlog::sinks::test_sink_st;
-    auto test_sink = std::make_shared<test_sink_st>();
-    size_t backtrace_size = 5;
-
-    spdlog::logger logger("test-backtrace", test_sink);
-    logger.set_pattern("%v");
-    logger.enable_backtrace(backtrace_size);
-    logger.dump_backtrace();
-    REQUIRE(test_sink->lines().size() == 0);
-}
-
-TEST_CASE("bactrace-async", "[bactrace]") {
-    using spdlog::sinks::test_sink_mt;
-    auto test_sink = std::make_shared<test_sink_mt>();
-    using spdlog::details::os::sleep_for_millis;
-
-    size_t backtrace_size = 5;
-
-    spdlog::init_thread_pool(120, 1);
-    auto logger = std::make_shared<spdlog::async_logger>("test-bactrace-async", test_sink,
-                                                         spdlog::thread_pool());
-    logger->set_pattern("%v");
-    logger->enable_backtrace(backtrace_size);
-
-    logger->info("info message");
-    for (int i = 0; i < 100; i++) logger->debug("debug message {}", i);
-
-    sleep_for_millis(100);
-    REQUIRE(test_sink->lines().size() == 1);
-    REQUIRE(test_sink->lines()[0] == "info message");
-
-    logger->dump_backtrace();
-    sleep_for_millis(100);  //  give time for the async dump to complete
-    REQUIRE(test_sink->lines().size() == backtrace_size + 3);
-    REQUIRE(test_sink->lines()[1] == "****************** Backtrace Start ******************");
-    REQUIRE(test_sink->lines()[2] == "debug message 95");
-    REQUIRE(test_sink->lines()[3] == "debug message 96");
-    REQUIRE(test_sink->lines()[4] == "debug message 97");
-    REQUIRE(test_sink->lines()[5] == "debug message 98");
-    REQUIRE(test_sink->lines()[6] == "debug message 99");
-    REQUIRE(test_sink->lines()[7] == "****************** Backtrace End ********************");
-}