fix(electrum): refresh header cache with headers fetched for the chain update

fix(electrum): refresh header cache with headers fetched for the chain update

`block_header_cache` was only written by `batch_fetch_anchors`, so after a
reorg it kept serving the pre-reorg header for that height. The anchor
cache is keyed by `(txid, hash)` from that header, so a tx re-mined at the
same height hit the anchor cache with the stale hash, skipped proof
validation, and was never re-anchored to the replacement block.

`fetch_tip_and_latest_blocks` now writes the headers it fetches (the
chain suffix and the agreement-walk checkpoints) into the header cache.