[codex] Add market data subscriber hub

[NewYaroslav]

Summary

• add IMarketDataSubscriber and MarketDataHub as an optional fan-out layer over provider callbacks
• cache and replay last stream status updates to late subscribers without replaying tick/bar payloads
• expose the hub through market_data.hpp, document the contract, and add CI coverage

Checks

• cmake -S . -B build-codex
• cmake --build build-codex --target market_data_hub_test -- -j1
• .\build-codex\market_data_hub_test.exe --gtest_brief=1
• .\build-codex\market_data_subscription_contract_test.exe --gtest_brief=1
• git diff --check