Prototype logging macro API split

[wgtmac]

Summary

This is a prototype for the logging macro API shape discussed in #725.

It demonstrates a possible end state:

• keep logger.h focused on the C++ logging API;
• move Iceberg-prefixed macros to log_macros.h;
• put short LOG_* aliases behind an explicit opt-in header;
• use preprocessor active-level gating so disabled fixed-level logs are fully compiled out;
• centralize formatting, fallback, runtime filtering, and fatal handling in shared helpers.

This is meant as a design reference, not a replacement for #725.

Test Plan

• .venv/bin/pre-commit run clang-format --files src/iceberg/logging/log_macros.h src/iceberg/logging/short_log_macros.h src/iceberg/logging/logger.h src/iceberg/test/log_macros_header_test.cc src/iceberg/test/log_macros_active_level_test.cc src/iceberg/test/log_macros_test.cc
• .venv/bin/pre-commit run cmake-format --files src/iceberg/CMakeLists.txt src/iceberg/test/CMakeLists.txt
• .venv/bin/pre-commit run meson-fmt --files meson.build src/iceberg/logging/meson.build src/iceberg/test/meson.build
• ninja -C builddir src/iceberg/test/logging_test
• ./builddir/src/iceberg/test/logging_test
• cmake --build build --target logging_test
• ./build/src/iceberg/test/logging_test