test: mutation testing for dev fee module

[mostronatorcoder]

Context

Follow-up from #618. Implement mutation testing for the dev fee module — handles revenue collection and payment processing.

Scope

• src/app/dev_fee.rs — Dev fee calculation, payment, and retry logic

Goal

• Run cargo mutants --file src/app/dev_fee.rs
• Document baseline mutation score
• Kill surviving mutants
• Target: >70% mutation score

Priority Mutants to Kill

• Fee percentage calculation
• Payment hash handling
• Stale pending marker cleanup
• Payment success/failure/timeout handlers
• dev_fee_paid flag transitions
• Claim/release optimistic locking

Notes

The dev fee module was recently refactored (#627) and has some tests already. This issue focuses on achieving comprehensive mutation coverage.

Acceptance Criteria

• Baseline mutation report
• Critical mutants in fee calculation killed
• Critical mutants in payment state transitions killed
• Mutation score documented in PR

Ref: #618