Execution Package

Canonical shared execution-kernel code lives here.

Modules

Module	Purpose
`__init__.py`	shared `Order`, `Fill`, `Trade`, and `ExecutionHandler`
`cost_model.py`	shared order-type execution cost profiles and rough-cost helpers
`order_book.py`	resting-order registry for the single-asset engine
`spread_model.py`	deterministic spread tick model
`time_controls.py`	session gating and trading-hour helpers

The portfolio engine reuses the shared fill semantics but still owns its own higher-level OMS bridge in portfolio_layer/.
Default retail execution cost profile:
- MARKET and STOP use the configured spread model.
- LIMIT and STOP_LIMIT default to zero spread slippage unless explicitly overridden.
- All order types fall back to the shared commission_rate unless explicitly overridden.