feat(execution): enforce min native reserve and ETH payout constraints

[rosspeili]

Summary

Implement native ETH payout path with reserve floor behavior.

Why

ADR/v1 scope requires native ETH handling and reserve semantics.

Scope

• Add reserve check logic
• Add ETH transfer handling
• Define behavior when reserve would be violated

Acceptance Criteria

• Reserve policy enforced in execution
• ETH transfer tests pass for success/failure paths