Build invoice payment network fee comparator

[Kingsman-99]

Label: complexity: high
Points: 200

Description

For payers choosing between direct token payment vs. pay_with_token/bridge_pay (DEX swap path), compare estimated total cost (resource fee + any swap slippage) across both paths to recommend the cheaper option.

Technical Context

New src/feeComparator.ts. Combines Issue 34's fee estimator with a swap-cost estimate (querying the configured DEX contract's quote, if available) to produce a side-by-side comparison.

Acceptance Criteria

• compareFundingPaths(invoice, sourceToken, amount) returns { direct: CostEstimate | "unsupported", swap: CostEstimate | "unsupported", recommended: "direct"|"swap" }
• Marks a path "unsupported" rather than erroring when, e.g., the source token isn't the invoice token and no DEX is configured
• Tests: both-supported comparison picks the cheaper path; one-path-unsupported case handled correctly

[Glam26]

I'd be happy to take this on.

I've worked on similar issues across multiple open-source and production projects, including feature development, refactoring, testing, documentation, performance improvements, and bug fixes. My approach is to first understand the existing architecture and conventions, then deliver a solution that is clean, maintainable, and aligned with the project's standards.

I'll make sure the implementation is thoroughly tested, well-documented where necessary, and covers relevant edge cases. I'm comfortable collaborating during review, incorporating feedback quickly, and ensuring the final result is production-ready.

[Glam26]

I'd be happy to take this on.

I've worked on similar issues across multiple open-source and production projects, including feature development, refactoring, testing, documentation, performance improvements, and bug fixes. My approach is to first understand the existing architecture and conventions, then deliver a solution that is clean, maintainable, and aligned with the project's standards.

I'll make sure the implementation is thoroughly tested, well-documented where necessary, and covers relevant edge cases. I'm comfortable collaborating during review, incorporating feedback quickly, and ensuring the final result is production-ready.