shell: keyword redirection + post-filter flags (ADR 0002)

Implement the shell experience redesign: one canonical grammar for every
command — 'cmd args [USE src] [TIL marker] [filters...] [output kw dst]'.

- Keyword redirection (uppercase-only, aliases valid, fully reserved):
  WRITING/WRITE overwrite, TO append, USE/USING input source,
  TIL/UNTIL heredoc/terminator (exact full-line marker).  > >> < << are
  removed; '>' is plain text.
- Post-filter stages applied left to right over a line stream, each
  repeatable: -h N (head kills the upstream), -t N (tail holds a
  rolling window, flushes at EOF/^C), -g PAT (smart-case literal
  substring), -p (screenful pager, terminal only).  A file sink
  replaces the screen; -p is a no-op with one.
- C-style quoting: "..." groups and literalizes, \x escapes one char;
  quoted/escaped text is never an operator, never split.
- Input consumers: cat/hexdump, path-less only (stdin until ^D,
  USE f, TIL m heredoc, USE f TIL m truncation).  Other commands
  reject input keywords; paths + input keyword is a syntax error.
- ^D is EOF everywhere and never powers off; poweroff/exit are the
  only shutdown verbs.
- help rewritten; test-disk.sh and sendkeys.py use the keyword syntax.

Verified in QEMU: 26 grammar checks on fresh and reused disks, disk
round trip, pager pacing, -h stopping /dev/mem at N lines, ^C flushing
a -t window.  Two bugs found during verification and fixed:
1. WRITING over an existing file wrote after stale NUL padding —
   fs::size() is stale after set_len(0); the sink writes at offset 0
   after truncation.
2. USE f TIL m leaked the post-marker remainder — the final
   partial-line emit ran after the marker break; guarded with a
   stopped flag.

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