Remove the dead BORG_TESTONLY_SHA256_PACK_ID tox env and CI job, fix the packs.rst pack-id docs, and reword comments/tests to describe sha256 pack naming instead of the removed shortcut.
Use BORG_OBJ (8 bytes) as the blob magic and refer to it as OBJ_MAGIC
throughout so the literal and its length appear in one place. Update
the inline blob diagram to the pipe-separated notation Thomas suggested.
Rename PackIndex->ChunkIndex, fix pack path to use pack_id, drop
levels_config detail, fix "keyed MAC"->"ID hash" in Pack ID section,
document chunk_id duplication across unencrypted header and encrypted_meta.
remove forward-looking N>1 references, hardcoded offsets, and stale "currently";
use borgstore vocabulary, medium-sized index files, and simplified recovery prose
