Implements the source PDF's prescribed model for directory structure:
'treat the CZYX cube as the only persistent store, with system calls that
operate on CZYX records and Null-space flags rather than paths and inodes.'
Nested dirs are RECONSTRUCTED from a per-record `path` metatag, so the
filesystem does not need recursive inode trees.
- cubecoords: add CubeHeader.path (Option<String>) + HAS_PATH flag bit (1<<8);
refresh_flags() sets it. Cubestore TLV codec now serializes/deserializes the
path as tag 13 (persists across checkpoint/reopen).
- cubestore: scan_by_path (exact), scan_by_path_prefix (dir listing by path
prefix; bare '/etc' normalized to '/etc/'), plus query_by_path/_prefix
returning decoded (coord,header,body).
- cubesys: `put` verb gains path=<p>;
new `query-path <dir>` verb reconstructs a directory listing from metatags.
- ConcurrentStore: query_by_path(_prefix) delegate to inner CubeStore.
Verified live in VM: migrated /etc/passwd, /etc/network/interfaces, /usr/bin/ls
at unrelated coordinates; 'query-path /etc' returns the two /etc files,
'/usr/bin/ls' correctly excluded. cubestore 8/8 tests pass.
Refs: OS-in-CUBE migration, query-layer prototype.
CUBELinux-2
94681bbdc0