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, 52 deletions

diff --git a/thirdparty/spdlog/tests/test_ringbuffer.cpp b/thirdparty/spdlog/tests/test_ringbuffer.cpp
deleted file mode 100644
index 81d791656..000000000
--- a/thirdparty/spdlog/tests/test_ringbuffer.cpp
+++ /dev/null
@@ -1,52 +0,0 @@
-#include "includes.h"
-#include "spdlog/sinks/ringbuffer_sink.h"
-
-TEST_CASE("ringbuffer invalid size", "[ringbuffer]") {
-    REQUIRE_THROWS_AS(spdlog::sinks::ringbuffer_sink_mt(0), spdlog::spdlog_ex);
-}
-
-TEST_CASE("ringbuffer stores formatted messages", "[ringbuffer]") {
-    spdlog::sinks::ringbuffer_sink_st sink(3);
-    sink.set_pattern("%v");
-
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "msg1"});
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "msg2"});
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "msg3"});
-
-    auto formatted = sink.last_formatted();
-    REQUIRE(formatted.size() == 3);
-    using spdlog::details::os::default_eol;
-    REQUIRE(formatted[0] == spdlog::fmt_lib::format("msg1{}", default_eol));
-    REQUIRE(formatted[1] == spdlog::fmt_lib::format("msg2{}", default_eol));
-    REQUIRE(formatted[2] == spdlog::fmt_lib::format("msg3{}", default_eol));
-}
-
-TEST_CASE("ringbuffer overrun keeps last items", "[ringbuffer]") {
-    spdlog::sinks::ringbuffer_sink_st sink(2);
-    sink.set_pattern("%v");
-
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "first"});
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "second"});
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "third"});
-
-    auto formatted = sink.last_formatted();
-    REQUIRE(formatted.size() == 2);
-    using spdlog::details::os::default_eol;
-    REQUIRE(formatted[0] == spdlog::fmt_lib::format("second{}", default_eol));
-    REQUIRE(formatted[1] == spdlog::fmt_lib::format("third{}", default_eol));
-}
-
-TEST_CASE("ringbuffer retrieval limit", "[ringbuffer]") {
-    spdlog::sinks::ringbuffer_sink_st sink(3);
-    sink.set_pattern("%v");
-
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "A"});
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "B"});
-    sink.log(spdlog::details::log_msg{"test", spdlog::level::info, "C"});
-
-    auto formatted = sink.last_formatted(2);
-    REQUIRE(formatted.size() == 2);
-    using spdlog::details::os::default_eol;
-    REQUIRE(formatted[0] == spdlog::fmt_lib::format("B{}", default_eol));
-    REQUIRE(formatted[1] == spdlog::fmt_lib::format("C{}", default_eol));
-}