[FIX] Order Entry session (prediction + perps) #424

Scope

FIX Order Entry (OE) message flow for both products over the shared session engine.

Messages

• Outbound: NewOrderSingle (35=D), OrderCancelRequest (35=F), OrderCancelReplaceRequest (35=G), OrderMassCancelRequest (35=q).
• Inbound: ExecutionReport (35=8), OrderCancelReject (35=9), OrderMassCancelReport (35=r), BusinessMessageReject (35=j).

Work

• Typed order vocabulary mapping to/from FIX: Side (54: BUY_YES/SELL_NO), OrdType (40: LIMIT), TimeInForce (59), SelfTradePreventionType (2964), ExecInst (POST_ONLY), OrderGroupID, MaxExecutionCost, AllocAccount.
• Money/qty precision: DollarDecimal for prices, FixedPointCount for quantities. Perps: fixed-point dollars (≤6dp) + FixedPointCount (≤2dp) with UseDollars on; prediction: integer cents unless UseDollars.
• NoPartyIDs/NoMiscFees/NoCollateralAmountChanges groups (depends on repeating-group support).
• Typed send helpers on FixClient/MarginFixClient and typed inbound delivery (ExecutionReport stream / callback).

Acceptance

Golden round-trip fixtures for every OE message; state tests for order lifecycle (New → PartiallyFilled → Filled, Cancel, Replace, Reject) against the mock acceptor. ruff + mypy --strict clean.

Dependencies

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