fix(chain)!: report `prev_blockhash` mismatches from `insert_block`

fix(chain)!: report `prev_blockhash` mismatches from `insert_block`

`insert_block` mapped every `apply_changeset` error to an
`AlterCheckPointError` about removing genesis. The only reachable error
there is a `prev_blockhash` mismatch, so callers inserting an unlinked
`Header` were told genesis could not be removed.

`insert_block` now returns `InsertBlockError`, which reports either the
existing conflicting block or the `prev_blockhash` mismatch. With no
users left, `AlterCheckPointError` is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>