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, 102 deletions

diff --git a/thirdparty/spdlog/tests/utils.cpp b/thirdparty/spdlog/tests/utils.cpp
deleted file mode 100644
index 405b5e5a2..000000000
--- a/thirdparty/spdlog/tests/utils.cpp
+++ /dev/null
@@ -1,102 +0,0 @@
-#include "includes.h"
-
-#ifdef _WIN32
-    #include <windows.h>
-#else
-    #include <sys/types.h>
-    #include <dirent.h>
-#endif
-
-void prepare_logdir() {
-    spdlog::drop_all();
-#ifdef _WIN32
-    system("rmdir /S /Q test_logs");
-#else
-    auto rv = system("rm -rf test_logs");
-    if (rv != 0) {
-        throw std::runtime_error("Failed to rm -rf test_logs");
-    }
-#endif
-}
-
-std::string file_contents(const std::string &filename) {
-    std::ifstream ifs(filename, std::ios_base::binary);
-    if (!ifs) {
-        throw std::runtime_error("Failed open file ");
-    }
-    return std::string((std::istreambuf_iterator<char>(ifs)), (std::istreambuf_iterator<char>()));
-}
-
-std::size_t count_lines(const std::string &filename) {
-    std::ifstream ifs(filename);
-    if (!ifs) {
-        throw std::runtime_error("Failed open file ");
-    }
-
-    std::string line;
-    size_t counter = 0;
-    while (std::getline(ifs, line)) counter++;
-    return counter;
-}
-
-void require_message_count(const std::string &filename, const std::size_t messages) {
-    if (strlen(spdlog::details::os::default_eol) == 0) {
-        REQUIRE(count_lines(filename) == 1);
-    } else {
-        REQUIRE(count_lines(filename) == messages);
-    }
-}
-
-std::size_t get_filesize(const std::string &filename) {
-    std::ifstream ifs(filename, std::ifstream::ate | std::ifstream::binary);
-    if (!ifs) {
-        throw std::runtime_error("Failed open file " + filename);
-    }
-    return static_cast<std::size_t>(ifs.tellg());
-}
-
-// source: https://stackoverflow.com/a/2072890/192001
-bool ends_with(std::string const &value, std::string const &ending) {
-    if (ending.size() > value.size()) {
-        return false;
-    }
-    return std::equal(ending.rbegin(), ending.rend(), value.rbegin());
-}
-
-#ifdef _WIN32
-// Based on: https://stackoverflow.com/a/37416569/192001
-std::size_t count_files(const std::string &folder) {
-    size_t counter = 0;
-    WIN32_FIND_DATAA ffd;
-
-    // Start iterating over the files in the folder directory.
-    HANDLE hFind = ::FindFirstFileA((folder + "\\*").c_str(), &ffd);
-    if (hFind != INVALID_HANDLE_VALUE) {
-        do  // Managed to locate and create an handle to that folder.
-        {
-            if (ffd.cFileName[0] != '.') counter++;
-        } while (::FindNextFileA(hFind, &ffd) != 0);
-        ::FindClose(hFind);
-    } else {
-        throw std::runtime_error("Failed open folder " + folder);
-    }
-
-    return counter;
-}
-#else
-// Based on: https://stackoverflow.com/a/2802255/192001
-std::size_t count_files(const std::string &folder) {
-    size_t counter = 0;
-    DIR *dp = opendir(folder.c_str());
-    if (dp == nullptr) {
-        throw std::runtime_error("Failed open folder " + folder);
-    }
-
-    struct dirent *ep = nullptr;
-    while ((ep = readdir(dp)) != nullptr) {
-        if (ep->d_name[0] != '.') counter++;
-    }
-    (void)closedir(dp);
-    return counter;
-}
-#endif