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)