Allow SFTP clients to reach the same logical paths as the shell (system
image, pseudo, home, mounts), deny mutating OPEN on typical read-only
prefixes, merge file/dir handle IDs to avoid collisions, implement FSTAT,
stat-based READDIR NAME attrs/longnames, and improve REALPATH.
Add brittle tests for path policy and wire test.bare-openssh-sftp.js into
npm test.
