v3.3.0

Adds a complete FIX protocol subsystem (FIXT.1.1 / FIX50SP2) for both the
prediction and perps products. Additive release: no changes to the existing REST,
WebSocket, or Perps surfaces.

Added

• FIX engine (kalshi.fix) — a hand-rolled, async-first FIX client for both
  products: FixClient (prediction) and MarginFixClient (margin, in
  kalshi.perps.fix), re-exported from the top-level kalshi package alongside
  FixConfig, FixEnvironment, and FixSessionType. Covers five session types —
  order entry (NR/RT), drop copy, market data, post-trade settlement (prediction),
  and RFQ (prediction), plus order-group management over the order-entry session —
  on a session state machine with
  RSA-PSS logon (reusing the REST key), heartbeat / test-request liveness,
  sequence tracking with gap-fill / resend on the retransmission sessions, and
  AWS-style full-jitter reconnect. (Epic #402.)
• Typed FIX messages — Pydantic v2 models for every Kalshi FIX message across
  order entry, order groups, market data, RFQ/quoting, drop copy, and market
  settlement, with DollarDecimal / FixedPointCount money types (no float
  drift) and a central inbound dispatch via decode_app_message.
• FixOrderBook — aggregated order-book reconstruction from market-data
  snapshots + incrementals; SettlementReassembler — paginated
  settlement-report reassembly.
• FixSession.on_decode_error hook + decode_app_message_strict — surface a
  registered-but-malformed inbound message instead of silently dropping it (#432).
• FIX documentation — new docs/fix.md guide, plus FIX coverage in the
  README, errors, authentication, configuration, environment-variables, and the
  API reference.
• FIX dictionary drift test — specs/kalshi-fix-dictionary.xml checked in
  with a model↔dictionary drift test, the FIX analogue of the REST contract-drift
  suite (#437).

Fixed

• Documentation accuracy pass: fcm.positions_all() is now documented (the FCM
  page previously stated it did not exist); the portfolio reads document their
  subaccount parameter; the markets.is_block_trade version note is corrected
  to SDK v3.1.0; the OrderPrice and MultiplierDecimal types are documented;
  and the docs landing page's WebSocket channel count is corrected to 11.