Moves the reachability walk onto btree-ondisk 0.20's BMap::collect_ptrs, which
owns the direct/B+tree distinction this crate used to branch on by hand. One
call replaces both arms — nonleafnode_iter for a B+tree bmap, and the
seek_key + lookup walk added in 0.4.0 for a direct one. collect_ptrs also
returns Err rather than a short list when a subtree cannot be enumerated, so an
unreadable metadata node is no longer indistinguishable from an absent
reference.

The bmap root handed to BMap::read is now aligned by construction, borrowed out
of an owned InodeRaw rather than copied out as a bare [u8; N].

Dependencies: hyperfile 0.6.10 -> 0.6.11, btree-ondisk ^0.18 -> 0.20.0. cargo
treats 0.x minors as incompatible, so downstream binaries linking another
btree-ondisk consumer must move together.

The public API is unchanged.