malkhut(wire): OrderType as three orthogonal dimensions — Fable's corrections

CRITICAL REFACTOR based on Fable's review (S9 roadmap item):

Before: flat enum conflating order types with TIF/instructions
  OrderType had MARKET, LIMIT, IOC, FOK, POST_ONLY, REDUCE_ONLY, etc.

After: three orthogonal dimensions (FIX-aligned):
  1. OrderType (Tag 40): what the order IS
     LIMIT, MARKET, STOP_MARKET, STOP_LIMIT, TRIGGER_MARKET, TRIGGER_LIMIT,
     TRAILING_STOP, OCO, TP_SL
  2. TimeInForce (Tag 59): how long it LIVES
     GTC, IOC, FOK, GTD
  3. Instructions (Tag 18): behavioral modifiers
     POST_ONLY, REDUCE_ONLY, HIDDEN, ICEBERG

Key corrections:
- POST_ONLY is an instruction on a LIMIT order, not a standalone type
- IOC/FOK are TimeInForce values, not order types
- BingX trailing_stop -> native TRAILING_STOP_MARKET (not TRIGGER_MARKET)
- FulfilmentAction.time_in_force: new field, default GTC

Exchange mappings restructured:
  EXCHANGE_ORDER_TYPE_MAP: OrderType -> exchange native 'type' param
  EXCHANGE_TIF_MAP: TimeInForce -> exchange native 'timeInForce' param
  EXCHANGE_INSTRUCTION_MAP: Instruction -> exchange encoding

21 files changed. 380+ tests pass. Backward compatible.

MALKHUT/malkhut/state.py

@@ -26,33 +26,23 @@ class Side(str, Enum):
 class OrderType(str, Enum):
     """Standardized order types — FIX/CCXT-aligned, multi-exchange.
 
-    Five-layer taxonomy (values are uppercase for backward compatibility):
-      Layer 1: Base types (FIX Tag 40) — MARKET, LIMIT
-      Layer 2: Time-in-force (FIX Tag 59) — GTC, IOC, FOK, GTD
-      Layer 3: Conditional (FIX Tag 3/4+MIT) — STOP_MARKET, STOP_LIMIT, etc.
-      Layer 4: Instructions (FIX Tag 18) — POST_ONLY, REDUCE_ONLY, etc.
-      Layer 5: Compound (exchange-specific) — OCO, TP_SL
+    THREE ORTHOGONAL DIMENSIONS (not one flat enum):
+      1. Order Type (FIX Tag 40): what the order IS — this enum
+      2. TimeInForce (FIX Tag 59): how long it LIVES — separate parameter
+      3. Instructions (FIX Tag 18): behavioral modifiers — separate parameter
+
+    CRITICAL: IOC, FOK, POST_ONLY are NOT order types.
+      IOC/FOK = TimeInForce on a LIMIT order.
+      POST_ONLY = ExecInst modifier on a LIMIT order.
     """
-    # Layer 1: Base types (FIX Tag 40)
+    # Core order types (FIX Tag 40)
     MARKET = "MARKET"
     LIMIT = "LIMIT"
-    # Layer 2: Time-in-force (FIX Tag 59)
-    GTC = "GTC"
-    IOC = "IOC"
-    FOK = "FOK"
-    GTD = "GTD"
-    # Layer 3: Conditional (FIX Tag 3/4 + MIT)
     STOP_MARKET = "STOP_MARKET"
     STOP_LIMIT = "STOP_LIMIT"
     TRIGGER_MARKET = "TRIGGER_MARKET"
     TRIGGER_LIMIT = "TRIGGER_LIMIT"
     TRAILING_STOP = "TRAILING_STOP"
-    # Layer 4: Instructions (FIX Tag 18)
-    POST_ONLY = "POST_ONLY"
-    REDUCE_ONLY = "REDUCE_ONLY"
-    HIDDEN = "HIDDEN"
-    ICEBERG = "ICEBERG"
-    # Layer 5: Compound (exchange-specific)
     OCO = "OCO"
     TP_SL = "TP_SL"