Function walk_btree 

pub fn walk_btree<F: FileBackend>(
    pager: &mut Pager<F>,
    ctx: &TreeContext,
    reachable: &mut HashSet<PageId>,
    failures: &mut Vec<IntegrityFailure>,
) -> Result<u64>

Walk a single B-tree from ctx.root, recording any per-node invariant violations into failures and inserting every visited node page-id into reachable. Returns the number of pages this walk inspected.

The walk traverses the tree level by level via an explicit Vec<PageId> queue and a HashSet<PageId> already-seen guard (so a cycle introduced by an out-of-band mutation cannot infinitely-loop the walker). Bounded by MAX_RANGE_NODES per tree (Rule 2).

Errors

Returns Error::Io on cache-miss read failure. Never returns an Error::Corruption directly — corruption surfaces as a IntegrityFailure entry in failures.