BTreeV1Node::parse_in reads a node's header and then its keys and
children, two bounded reads; collect_symbol_table_nodes_in walks the tree
over any Storage. The &[u8] functions are wrappers. New test: nodes with
siblings and 4- and 8-byte offsets cut at every length, and a two-level
tree with truncated leaves, give identical results through a read_at-only
CountingStorage (six reads for the three nodes).
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>