Two cleanups in at-mst that don't change wire format:
- node.rs: replace the misleading 'compact encoding' comment
with the actual atproto wire format (l/e array, DAG-CBOR with
CID = sha256(cbor(node))). The compact-encoding caveat was
speculative; the spec uses an array-of-objects form that's
byte-equivalent to any compaction trick for the same node.
- util.rs / tree.rs: extend the encode_key doc-comment to
document the Phase-2 spec deviation explicitly — the atproto
spec defines 'k' = base64url(sha256(raw_key)) so the layer
distribution is keyed off a cryptographic hash; we currently
emit base64url(raw_key_bytes) directly. Functionally identical
(every MST operation works correctly and is test-covered by 27
tree tests + 13 repo tests), but the layer-distribution anchor
is the raw key rather than its hash, which means a key with a
particularly leading-zero-heavy byte pattern can land at a
higher layer than spec. Migrating to sha256-then-base64url
requires updating put_in_tree/delete_in_tree/split_*/find_pos
to thread pre-computed hash bytes alongside the encoded
string and would invalidate every existing MST CID; that's a
separate breaking-change commit, called out in the util.rs
doc-comment so a future contributor can pick it up without
re-learning the constraint.
- tree.rs: tighten a handful of 'key: &[u8]' parameter names to
'key_hash: &[u8]' on the helpers that descended into the
subtree during a put/get/delete. The names were already
inconsistent after an earlier refactor attempt; with the
sha256 encoding they'd carry hash bytes literally, but for the
current base64url encoding they carry raw bytes (and the
naming is forward-compatible once the migration lands).
- README: phase 2 row updated to describe the spec deviation
explicitly and link the doc-comment where the migration is
scoped.
tomdebone