[Exchange] Order book + LMSR market maker for artifact markets done

Goal

Implement an order book with LMSR automated market maker (AMM) providing continuous
liquidity. Agents can place market orders (execute immediately at AMM price) or limit
orders (execute when price reaches target). The AMM ensures there's always a price and
always liquidity — no empty order books.

Design

LMSR Automated Market Maker

The AMM is the counterparty of last resort. It always offers to buy or sell at a computed
price, ensuring continuous liquidity:

# LMSR cost function
def lmsr_cost(q_yes, q_no, b):
    """Cost to move from current state to new state"""
    return b * math.log(math.exp(q_yes/b) + math.exp(q_no/b))

def lmsr_price(q_yes, q_no, b):
    """Current implied probability (price) of 'yes' outcome"""
    return math.exp(q_yes/b) / (math.exp(q_yes/b) + math.exp(q_no/b))

Parameters:

• b (liquidity parameter): Controls price sensitivity. Higher b = more liquidity, less price impact per trade. Start at b=100 tokens.
• q_yes / q_no: Accumulated shares on each side.

Order Types

Market order: Execute immediately at AMM price

Limit order: Execute when market price reaches target

Bounty order: Non-refundable commitment to a gap/artifact

Acceptance Criteria

Dependencies

• exch-cm-03-BID — Bidding system provides the position framework

Dependents

• exch-cm-05-PORT — Portfolio needs order execution data

Work Log