[codex] Add MetaTrader file bridge

[NewYaroslav]

Summary

Adds the first real MetaTrader Common Files bridge implementation on top of the NDJSON file transport foundation.

What changed

• add MetaTraderFileBridge as the runtime bridge class exposed through bridges/metatrader_file.hpp
• poll commands.ndjson with bounded sequence windows and persist commands.checkpoint.json
• handle protocol.hello, account.balance.get, signal.submit, and trade.open
• publish JSON-RPC responses plus balance.updated and trade.updated notifications to events.ndjson
• keep state snapshots in state.json
• add polling batch limits to MetaTraderFileBridgeConfig
• move bridge-specific account/report/signal JSON helpers into detail/MetaTraderFileBridgeUtils.hpp
• add metatrader_file_bridge_smoke example with --self-test and wire it into Ubuntu/Windows smoke CI
• cover signal intake, account query, trade update events, and umbrella include exposure

Validation

• cmake --build build-codex-pr68-mingw --target metatrader_file_bridge_test bridge_umbrella_include_test metatrader_file_config_include_test metatrader_paths_test -j2
• build-codex-pr68-mingw\\metatrader_file_bridge_test.exe --gtest_brief=1
• build-codex-pr68-mingw\\bridge_umbrella_include_test.exe --gtest_brief=1
• build-codex-pr68-mingw\\metatrader_file_config_include_test.exe --gtest_brief=1
• build-codex-pr68-mingw\\metatrader_paths_test.exe --gtest_brief=1
• cmake -S . -B build-codex-examples-mingw -DOPTIONX_BUILD_DEPS=ON -DOPTIONX_BUILD_EXAMPLES=ON -DOPTIONX_BUILD_TESTS=OFF -DOPTIONX_LIGHTWEIGHT_BRIDGE_SMOKE_TESTS=ON
• cmake --build build-codex-examples-mingw --target metatrader_file_bridge_smoke -j2
• build-codex-examples-mingw\\metatrader_file_bridge_smoke.exe --self-test
• git diff --check