fix(logging): harden async logger shutdown

[LimiNode]

Summary

• stop logger-owned async workers through shutdown() and reject direct backend logs after shutdown
• close Logger::shutdown() races for add/dispatch and harden MPSC hot resize against active producers
• fix MPSC TaskExecutor::wait() so it cannot return between dequeue cell release and task execution
• serialize OTLP sync exports with shutdown and keep async Block producers wakeable during stop
• clean OTLP include policy and update TaskExecutor lifecycle documentation
• add regression tests for backend terminal shutdown, Logger shutdown races, TaskExecutor resize races, and Emscripten SingleThreadExecutor policies

Test Plan

• git diff --check -- . ':!external/zlib'
• g++ -std=c++17 -Iinclude/logit_cpp -Iexternal/time-shield-cpp/include tests/raw_file_logger_test.cpp -o tmp/codex_verify/raw_file_logger_test.exe + run
• g++ -std=c++11 -Iinclude/logit_cpp -Iexternal/time-shield-cpp/include tests/backend_shutdown_terminal_test.cpp -o tmp/codex_verify/backend_shutdown_terminal_test.exe + run
• g++ -std=c++11 -Iinclude/logit_cpp -Iexternal/time-shield-cpp/include tests/logger_shutdown_race_test.cpp -o tmp/codex_verify/logger_shutdown_race_test.exe + run
• g++ -std=c++11 -Iinclude/logit_cpp -Iexternal/time-shield-cpp/include tests/task_executor_resize_race_test.cpp -o tmp/codex_verify/task_executor_resize_race_test.exe + run
• OTLP header compile probe via <logit.hpp> with a temporary local mock kurlyk.hpp, then run
• earlier: cmake -S . -B build-codex -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Debug
• earlier: cmake --build build-codex -j 4 (blocked only by unrelated untracked tests/test_mdc_ndc.cpp, which references MDC/NDC macros absent from this branch)
• earlier: ctest --test-dir build-codex --output-on-failure -E test_mdc_ndc

Notes

• Emscripten-specific tests were expanded but not run locally because emcmake / em++ were unavailable on this machine.
• Full local CMake regeneration is still affected by unrelated local artifacts/configuration: untracked tests/test_mdc_ndc.cpp is picked up by the test glob, and the existing build-mingw directory fails regeneration on a zlib export-set issue.