Add median-time-past (MTP) calculation to CheckPoint

[evanlinjin]

Summary

Add median-time-past (MTP) calculation capability to CheckPoint for time-based validation in Bitcoin consensus rules.

Proposed Changes

1. Extend ToBlockHash trait with optional block time:

fn block_time(&self) -> Option<u32> { None }

2. Add median_time_past() method to CheckPoint:
   • Calculate median of up to 11 previous block timestamps
   • Return None if:
     • Data type doesn't support block times
     • Required blocks are missing (sparse chain)
     • Insufficient block history available

Use Cases

• Transaction time-lock verification (nLockTime, OP_CHECKLOCKTIMEVERIFY)
• BIP113 compliance

Notes

• CheckPoint represents a sparse chain, so required blocks may not be available
• MTP = median of current block + up to 10 predecessors (sorted timestamps)

[Serious-Sam-Dev]

I have a little question, have a better form to get the prev block in the checkpoint without use checkpoint.prev()? I'd like to work on this feature, but you know, i am new hear. Maybei have another form to make this most easy and i dont know

[evanlinjin]

@Serious-Sam-Dev thank you for asking your little question.

Note that checkpoints may be sparse. .prev may not get the previous adjacent block. It'll get the previous checkpoint.

You can iterate checkpoints with .iter.

You're new, I hear you. Note that we already have a PR addressing this issue. Reviewing that would be helpful.