fix(market-data): chunk initial tick prefill

[LimiNode]

Summary

• bound initial tick prefill by max_backfill_ms`n- continue inclusive chunks with provider-grid overlap until the prefill boundary
• keep buffered live ticks until the final chunk and preserve restart-to-current-boundary semantics
• add normal and interrupted chunked-prefill regression tests
• document the contract in EN and RU guides

Verification

• market_data_tick_continuity_test: 18/18
• market_data_tick_history_contract_test: 8/8
• market_data_router_test: 23/23
• market_data_subscription_contract_test: 26/26
• market_data_hub_test: 4/4
• market_data_continuity_test: 46/46
• git diff --check: passed

Stacked on #166 (base branch: ix/tick-continuity-dedup).