Have MTP in checkpoint data and report missing blocks

[evanlinjin]

Description

Context: #2076 (comment)

Adds a way to carry a precomputed median-time-past (MTP) value on each checkpoint at the type level, and reworks compute_mtp so callers can drive fetch-retry loops when blocks are missing.

• WithMtp<D> wrapper: CheckPoint<WithMtp<D>> gives a compile-time guarantee that MTP is available for every checkpoint, without walking 11 ancestor blocks per lookup. A new CheckPoint::mtp() method returns the stored value in O(1).
• compute_mtp returns Result<u32, MissingBlocks> instead of Option<u32>. MissingBlocks lists every height in the MTP window (h-10..=h) that the chain is missing, so callers — especially light-client chain sources like Electrum that don't expose MTP directly — can batch-fetch exactly the headers they need and retry.
• Rename median_time_past → compute_mtp to make the chain-walk cost explicit and contrast with the O(1) mtp() accessor.

Notes to the reviewers

Breaking change, marked feat(core)!. Two API breaks:

• median_time_past → compute_mtp
• compute_mtp returns Result<u32, MissingBlocks> (was Option<u32>)

Changelog notice

Added:
- `WithMtp<D>` wrapper for checkpoint data with precomputed MTP.
- `CheckPoint::mtp()` on `CheckPoint<WithMtp<D>>` — O(1) MTP accessor.
- `MissingBlocks` error type.

Changed:
- `CheckPoint::median_time_past` renamed to `CheckPoint::compute_mtp`; return type changed from `Option<u32>` to `Result<u32, MissingBlocks>`.

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature