Cross-Chain Hub Improvements

[doitian]

I use this issue to trace recent improvements to the cross-chain hub.

There are three major features:

1. Integrate fiber invoices and payments in cross-chain hub
2. Persistent cross-chain hub orders and resume orders processing on restart
3. Allow running cross-chain hub in a separate process on a different host

@contrun started the work in the middle of 2025. He submitted #511 for 1, and #522 for 2 and 3. Then the work has been transferred to me. The first attempt is rebasing these PRs. The outcome:

• feat: support fiber multiple hops payment in cross chain transaction #615 for 1
• feat: cch persistence #898 for 2
• feat: cch separate service #901 for 3

As mpp has been added, #615 can be further simplified, so I decide to split it into small PRs to ease reviewing. Here is the plan:

• Add feature hold invoice which has no preimage. The TLC is held and RPC settle_invoice is added to settle the held TLCs. feat: hold invoice #945
• Add mpp support to hold invoice. (test case added in the second commit of feat: hold invoice #945)
• Add fiber invoices and payments support in cch feat: multi-hop fiber payments for cch #942
• Allow subscribing store change events feat: record and notify store changes #930
• Use store change event to monitor invoices and payments changes refactor(cch): refactor lnd trackers #948 feat: monitor fiber store changes for cch #950
• Refactoring Refactor CCH order with FSM and actions #971
• More verifications feat(cch): add expiry, preimage, and invoice validations #998
• Persistent cross-chain hub orders and resume orders processing on restart feat(cch): implement order persistence and housekeeping #1045
• Allow running cross-chain hub in a separate process on a different host feat: cch separate service #901