Merge bitcoindevkit/bdk#2381: docs(core): require `CheckPoint` data to be determined...

Merge bitcoindevkit/bdk#2381: docs(core): require `CheckPoint` data to be determined by its block hash

fbe1aa6335276b86f6d35fa409f473f8c797e02f docs(core): require `CheckPoint` data to be determined by its block hash (志宇)

Pull request description:

  ### Description

  The `ToBlockHash` docs said extra fields the block hash doesn't commit to "must be ignored", implying they're allowed. They aren't: `CheckPoint` and `LocalChain` treat data with equal hashes as interchangeable. The docs now require checkpoint data to be fully determined by its block hash, with anything else stored outside the checkpoint.

  The `BlockWithTime` test helper now commits to its time field.

  ### Changelog notice

  ```
  ### Changed
  - Documented that `CheckPoint` data must be fully determined by its block hash.
  ```

  ### Checklists

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  🤖 Generated with [Claude Code](https://claude.com/claude-code)

ACKs for top commit:
  oleonardolima:
    utACK fbe1aa6335276b86f6d35fa409f473f8c797e02f

Tree-SHA512: b9082833ce10149ac8be4a082537e542b59dd3748c65b6bed6912486d1ecc664bbc88966775c78db3d5cd9050e48b0a56078b775505a147cd90c0d351dd6aa27