shell: cat <path> TIL <marker> sugar + precise ordering errors

- `cat/hexdump <path> TIL <marker>` is now sugar for `USE <path> TIL
  <marker>`: the paths are the input source, truncated at the first
  marker line across the combined stream.  Path args + USE stays a
  syntax error (two input sources), now with a message that says so.
- The parser's `trailing` flag conflated filters with the output
  keyword, so `cat f TO out TIL m` blamed the filters.  USE/TIL now
  distinguish "before the filters" from "before the output keyword",
  and filters after the output keyword (`TO out -h 2`) are now
  rejected instead of silently accepted.
- cat_file_until/hexdump_file_until return a tri-state (Done/Marker/
  Aborted) so multi-path TIL truncation stops at the first marker.

Verified in QEMU (10 checks): truncation, combined-stream marker,
filters after the sugar, heredoc round trip, all four ordering-error
messages, USE-with-paths ambiguity, fsck clean.

0 files changed, 0 insertions, 0 deletions