[codex] Split MarketDataRouter public headers

[LimiNode]

Summary

• split the public Router identifiers into MarketDataRouterIds.hpp
• split the move-only RAII handle into MarketDataRouterSubscription.hpp
• keep market_data.hpp as the stable aggregate include and leave Router state implementation private in .ipp
• consolidate duplicated tick/bar subscription lifecycle and batch delivery logic in detail helpers

Verification

• market_data_router_test: 21/21 passed
• market_data_hub_test: 4/4 passed
• market_data_subscription_contract_test: 22/22 passed
• market_data_subscriber_base_test and example passed in Ubuntu CI
• Router example passed in Ubuntu CI
• g++ -std=c++17 -Iinclude -fsyntax-only examples/market_data_router_example.cpp
• git diff --check

The local SubscriberBase build was initially blocked by a mixed local dependency artifact set; clean Ubuntu and Windows CI passed the complete affected suite.