feat: onchain payments

[ye0man]

Summary

Add on-chain payments as an experimental mint and melt method in Coco.

This should follow the same generic method-handler architecture used
for Bolt11 and the planned Bolt12 support.

Status

This should wait until Bolt12 support is completed first: #160.

On-chain payment support should also be treated as experimental
because the underlying spec has not been merged yet. The
implementation and public API may need to change as the spec evolves.

Goal

Support on-chain quote creation, tracking, execution, refresh/
recovery, and settlement for both:

• mint operations
• melt operations

Existing Bolt11 behavior should remain unchanged, and this work should
build on the generic mint/melt method model rather than adding a
separate special-case flow.

Scope

• Core: add experimental on-chain mint/melt method data, handlers,
  validation, quote-state handling, and recovery behavior.
• Persistence: persist on-chain operations and quote metadata without
  assuming the shape is final.
• React: expose on-chain-capable mint/melt flows through the same
  operation APIs/hooks once core support is ready.
• Docs/tests: mark the feature as experimental and cover at least
  basic mint/melt flows.

Acceptance criteria

• On-chain mint and melt operations can be initiated through Coco’s
  generic operation APIs.
• The feature is clearly marked experimental / subject to spec
  changes.
• Bolt11 remains compatible and unchanged.
• The work is sequenced after Bolt12 support lands.

[ye0man]

we can prioritize this after #160