v2.2.0

Response-side spec drift hardening stack (#157). Backfills the remaining
v3.18.0 OpenAPI / v0.14 AsyncAPI fields across REST and WebSocket response
models (65 new optional fields across 16 models), promotes additive
drift from warning to hard CI failure, and lands a high-signal bugfix
for the subaccount-number request constraint that was rejecting valid
server-assigned values.

Added

• Market gains 11 spec fields, Order gains 8, Fill gains 4 (#159).
• Event gains 3, EventMetadata 2, Settlement 1, Trade 2,
  IncentiveProgram 1 (#160).
• RFQ gains 1, Quote 3, OrderGroup 1, GetOrderGroupResponse 1,
  CreateOrderGroupResponse 2 (#161).
• 33 fields across 11 WebSocket payload models (#162) — Unix-ms
  timestamps (*_ts_ms), outcome_side / book_side direction
  encoding, MVE linkage, and RFQ/Quote context echoes.
• ErrorPayload registered in WS_CONTRACT_MAP so WS contract coverage
  is complete.

Changed

• Response-side additive spec drift now hard-fails CI (was a
  warning). Closes the regression path that allowed
  Balance.balance_dollars to ship missing in v2.1.0 across five
  rounds of review. Intentional deviations require an entry in
  EXCLUSIONS (tests/_contract_support.py) with a typed kind and
  reason.
• Unmapped SDK models (REST + WS) also now hard-fail — same regression
  surface.
• WS envelope and helper models now use extra="allow", matching the
  WS payload policy from #143. Closes the matching ROADMAP item.
• Required-but-Optional drift stays as warning-only (~204 entries;
  separate policy decision).

Fixed

• Subaccount-number request fields no longer cap at 32. Demo allocates
  ephemeral subaccount numbers above 32 (observed: 41), but the SDK was
  rejecting them client-side with a ValidationError because seven
  request-side model fields carried a le=32 bound derived from spec
  prose. The actual OpenAPI schema defines no upper bound; only the
  description text mentions 1-32. Affected fields:
  ApplySubaccountTransferRequest.{from,to}_subaccount,
  UpdateSubaccountNettingRequest.subaccount_number,
  CreateOrderRequest.subaccount,
  BatchCancelOrdersV2RequestOrder.subaccount,
  CreateRFQRequest.subaccount,
  CreateQuoteRequest.subaccount.
  The ge=0 lower bound is unchanged. Unblocks the
  test_transfer_between_subaccounts nightly integration test (#164).