Tidy examples and cover classic expiry arithmetic

[LimiNode]

Summary

• add a pure fixed-value unit test for Intrade CLASSIC expiry arithmetic
• move generic example helpers into examples/example_utils.hpp
• trim duplicated helper code from smoke examples and add short orientation comments
• update examples README to describe the shared helper split

Validation

• git diff --check
• cmake --build build-codex-pr111-mingw --target intrade_bar_api_response_test --parallel 1
• .\build-codex-pr111-mingw\intrade_bar_api_response_test.exe --gtest_brief=1
• cmake --build build-codex-pr111-mingw --target bot_binary_command_builder_smoke metatrader_file_command_writer_smoke metatrader_file_end_to_end_smoke metatrader_file_bridge_smoke tradingview_extension_bridge_smoke intrade_market_data_example trade_record_db_example --parallel 1 (timed out during a buffered batch build; rerun target-by-target below)
• cmake --build build-codex-pr111-mingw --target bot_binary_command_builder_smoke --parallel 1
• cmake --build build-codex-pr111-mingw --target metatrader_file_command_writer_smoke --parallel 1
• cmake --build build-codex-pr111-mingw --target metatrader_file_end_to_end_smoke --parallel 1
• cmake --build build-codex-pr111-mingw --target metatrader_file_bridge_smoke --parallel 1
• cmake --build build-codex-pr111-mingw --target tradingview_extension_bridge_smoke --parallel 1
• cmake --build build-codex-pr111-mingw --target intrade_market_data_example --parallel 1
• cmake --build build-codex-pr111-mingw --target trade_record_db_example --parallel 1
• cmake --build build-codex-pr111-mingw --target bridge_umbrella_include_test --parallel 1
• .\build-codex-pr111-mingw\bot_binary_command_builder_smoke.exe --self-test
• .\build-codex-pr111-mingw\metatrader_file_command_writer_smoke.exe --self-test
• .\build-codex-pr111-mingw\metatrader_file_bridge_smoke.exe --self-test
• .\build-codex-pr111-mingw\metatrader_file_end_to_end_smoke.exe --self-test
• .\build-codex-pr111-mingw\tradingview_extension_bridge_smoke.exe --self-test