Pure tree walk: root → lanes → cards, level is position. Fail-fast with
path+reason for bad YAML, missing/malformed schema or order, newer
schema, rootless board; index-less folders skip with a collected+logged
warning; strays, hidden files, and symlinks ignored; board-level
deleted ignored with a warning; tombstones loaded and flagged. 17 tests.
Claude-Session: https://claude.ai/code/session_018BjQRYBR6jQja3jCRi5S3A
