docs(chain): document that MTP needs a contiguous 11-block window

docs(chain): document that MTP needs a contiguous 11-block window

An MTP at height `h` medians the timestamps at `h-10..=h`, so any height
in that window the driver cannot resolve makes the whole result `None` —
silently, since a gap is indistinguishable from a reorged-out block.

This is the normal case for a `LocalChain` built by syncing: electrum and
esplora insert a handful of checkpoints, not every height, so such a
chain yields `None` for both `prev_mtp` and `tip_mtp` until those clients
fetch the preceding window for each confirmed height.

Document the requirement on `with_mtp` and pin the behaviour with a test
over a chain holding only heights {0, 50, 100}, which also shows that the
gaps do not affect canonicalization itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NND8BhEmYTYdLRv4dbUUro