vfs: everything is a file — dirs, ramfs, character devices

- fs.rs: node-table VFS (root /dev /tmp /etc).  Regular files are
  in-memory ramfs; char devices regenerate content on every read so
  'cat /dev/rtc' shows the live time.  Nodes are stable handles with
  tombstone removal; . and .. path resolution.
- /dev devices: mem (raw physical RAM, offset = address), rtc, pci,
  version, meminfo, null, zero, console (write prints to screen+serial),
  serial.  /etc/motd static file.
- shell: ls, cat, hexdump (hexdump -C style), mkdir, touch, rm; echo
  gains '>' overwrite and '>>' append redirection (append bug fixed:
  first '>' in '>>' was parsed as overwrite)
- boot: fs init + self-test (create/read/write/remove) before the shell
- QEMU verified: file lifecycle, redirection, console/null writes,
  mkdir/touch/rm, hexdump /dev/mem and /tmp/x byte-exact

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