[FIX] Market Settlement / Post-Trade (prediction) #429

Scope

FIX Market Settlement / Post-Trade — prediction only. Read-only settlement-report stream on KalshiPT (port 8231), plus the ReceiveSettlementReports opt-in on KalshiRT.

Messages

MarketSettlementReport (UMS) with the nested NoMarketSettlementPartyIDs group (containing NoCollateralAmountChanges + NoMiscFees).

Work

• Typed MarketSettlementReport model incl. nested groups; tags 20105–20110, MarketResult (20107: yes/no/scalar), SettlementPrice (730), ClearingBusinessDate (715), LastFragment (893).
• PT session wiring + the ReceiveSettlementReports (20127) logon option (RT/PT only; default on PT).
• Multi-fragment report reassembly.

Acceptance

Golden fixtures for UMS incl. the nested-group decode and multi-fragment reassembly; PT session stream test against the mock acceptor. ruff + mypy --strict clean.

Dependencies

Foundation #422; repeating-group support (nested). Part of #402.