[RFC] Specify multi-leg execution state machine and recovery matrix

[brokermr810]

Parent epic

#254

Deliverable

Write an architecture RFC for a durable multi-leg execution state machine and its recovery behavior.

The RFC should define:

• Parent intent and child leg states, identifiers, and invariants.
• Sequential and parallel submission policies.
• Timeout, cancel, retry, compensation, and operator-intervention behavior.
• Partial-fill and unknown broker-state reconciliation.
• Restart and duplicate-delivery recovery.
• Broker capability boundaries, including venue-native combination orders.

Acceptance criteria

• A state diagram and transition table are included.
• First-leg failure, later-leg failure, partial fill, timeout, crash, and duplicate delivery are covered.
• Unmatched exposure can never be silently discarded.
• Idempotency and observability requirements are explicit.
• Deterministic paper-adapter test scenarios are proposed.
• The RFC is submitted as a focused draft pull request.

How to claim

Comment with a short outline and any prior work you plan to reuse. A maintainer will confirm the scope and assign the issue before implementation begins.