Fix LogLevel ordering so Warn threshold correctly suppresses Info messages

[Copilot]

LogLevel had Warn=1, Info=2, so shouldLog() (using >=) would pass Info messages through a Warn threshold and block Warn messages when the threshold was Info — the opposite of correct behaviour.

Changes

• src/logger.hh (new): defines tinyusdz::logging::LogLevel with monotonically-increasing severity values, a Logger singleton (default threshold: Warn), and TUSDZ_LOG_D/I/W/E/C macros with file/func/line context. Production builds (TINYUSDZ_PRODUCTION_BUILD) omit file paths.

  enum class LogLevel {
    Debug    = 0,
    Info     = 1,  // was 2 — caused Info to bypass Warn threshold
    Warn     = 2,  // was 1
    Error    = 3,
    Critical = 4,
    Off      = 5
  };

• tests/unit/unit-logger.cc (new): unit tests covering enum ordinal values and shouldLog() correctness across all threshold levels (Debug, Info, Warn, Off).

• tests/unit/unit-main.cc, CMakeLists.txt: wire logger_test into the existing unit-test binary.