Add fill-boundary C++ batch primitive

[tradingexpert]

Summary

• add a C++ batch primitive that advances compiled market-data rows until the first passive fill boundary
• expose the primitive through CppMatchingEngine.apply_events_until_fill(...)
• document how this supports future boundary-batched replay without changing audited Replay semantics yet

Tests

• python -m ruff check .
• python -m pytest
• mkdir -p build/cpp && c++ -std=c++17 -Wall -Wextra -pedantic -Icpp cpp/tests/matching_engine_core_test.cpp -o build/cpp/matching_engine_core_test && build/cpp/matching_engine_core_test

Coverage: 98.11% total.