[FIX] Repeating-group support in the typed message framework #423

Scope

Extend the typed FIX message framework (kalshi/fix/messages/base.py) and the codec to support repeating groups, which the foundation deliberately deferred (scalar fields only). Prerequisite for Order Entry, Market Settlement, and RFQ message families.

Work

• FixMessage base: declare and (de)serialize repeating groups in order, driven by the NoXxx count field + ordered member fields.
• Codec/RawMessage: ordered group extraction from the flat tag list (groups are order-sensitive and can nest).
• Groups needed across phases: NoPartyIDs (453), NoMiscFees (136), NoCollateralAmountChanges (1703), NoMultivariateSelectedLegs (20181), NoMarketSettlementPartyIDs (20108, nested).

Acceptance

• Golden round-trip fixtures for each group (encode → bytes → decode → model equality), including a nested group.
• ruff + mypy --strict clean; tests in tests/fix/.

Dependencies

Builds on the foundation (#422). Part of #402.