pipe: helix's other three shell commands, and its newline rule

`|` was the only one of helix's five. The other four differ in exactly two
things — whether the selection is stdin, and where the output lands — so they
are one action carrying a `PipeBehavior` rather than four code paths:

    |     stdin is the selection, output REPLACES it        (had this)
    A-|   stdin is the selection, output discarded          shell_pipe_to
    !     no stdin, output inserted BEFORE each selection   shell_insert_output
    A-!   no stdin, output appended AFTER each selection    shell_append_output

Each arms the same visible tag-tail prompt with its own marker (`|`, `|-`, `!`,
`!+`) so the prompt says which one you are in — they take the same command line
and do very different things to the buffer.

Two helix rules came with them. A behaviour that sends no stdin runs the
command ONCE and every cursor gets that one answer (helix's `shell_output`
cache): ten cursors and `date` give ten identical stamps rather than ten forks
racing to produce one. And a command that put a trailing newline on a selection
which did not have one has it taken back off — that is what keeps a one-line
`| tr a-z A-Z` from becoming two lines. The existing multi-range test moved
with that rule and now pins it deliberately.

In all three writing behaviours the OUTPUT is what ends up selected, keeping
the original range's direction, so an operator can follow straight on from what
the command just produced.

`$` (`shell_keep_pipe` — drop the selections whose command exited nonzero) is
still missing: it needs a per-selection verdict and the runner's answer is
atomic. Noted in docs/helix-keys.md beside the `$` divergence already there.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_016Q4RATpafkwahrovHQLKRf

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/helix-keys.md b/docs/helix-keys.md
index 81bc0248..048d8397 100644
--- a/docs/helix-keys.md
+++ b/docs/helix-keys.md
@@ -94,7 +94,7 @@ language-backend queries, and the shell pipe.
 | `w` `b` `e` | select to next word start / prev word start / next word end | full helix model incl. landing conventions (block cursor one before the next word after `w`) and newline/punct/EOF edges | helix-verified |
 | `W` `B` `E` | long-word (WORD) variants | same | helix-verified |
 | `Home` / `End` | line start / line end | matches `goto_line_start` / `goto_line_end` | helix-verified |
-| `0` / `$` / `^` | line start / line end / first non-ws | `0` and `^` are pardes extras — helix leaves those two unbound (it spells them `gh`/`gs`). `$` is NOT unbound in helix: it is `shell_keep_pipe` (`keymap/default.rs`), and taking it for line-end is a deliberate divergence — see the `$` row in C. `0` is a count digit while a count is pending | pardes-specific (`$`: deliberate) |
+| `0` / `$` / `^` | line start / line end / first non-ws | `0` and `^` are pardes extras — helix leaves those two unbound (it spells them `gh`/`gs`). `$` is NOT unbound in helix: it is `shell_keep_pipe` (`keymap/default.rs`), and taking it for line-end is a deliberate divergence — see the `$` row in C. It is the one shell command still unimplemented: it needs a per-selection verdict, where the runner's answer is atomic. `0` is a count digit while a count is pending | pardes-specific (`$`: deliberate) |
 | `G` | bare `G` is a **no-op**; `<n>G` = goto line n | vim-ism removed (phase 5): helix `goto_line` only acts with a count; `ge` is goto-last-line | helix-verified |
 | `gg` / `<n>gg` | goto first line / line n | | helix-verified |
 | `ge` | goto last content line | matches `goto_last_line` (ignores the trailing empty line) | helix-verified |
@@ -137,6 +137,9 @@ language-backend queries, and the shell pipe.
 | `=` | `format_selections` — writes a `- old` / `+ new` diff into `+Lsp` | deliberate divergence: the seam returns ROWS, not edits, so this SHOWS the formatting instead of applying it. Not in the corpus, so there is no waiver to name — the query leaves the core as an effect the headless harness has no shell to perform | out of corpus |
 | `Ctrl-o` / `Ctrl-i` | jumplist back / forward — the `Back` / `Forward` builtins, also on `SPC j o` / `SPC j i`, with `SPC j l` rendering the stack as a buffer | helix binds both keys (`jump_backward` / `jump_forward`) but to a POSITION jumplist; pardes' stack is over panes and focus, so the keys agree and the semantics do not. `Ctrl-i` and Tab are the same byte under the legacy encoding; there Tab keeps meaning execute, and the pair only separates where the host speaks the kitty keyboard protocol | pardes-specific |
 | `\|` | pipe every selection through `/bin/sh -c`: its bytes in on stdin, its stdout replacing them, one undo across all cursors | helix's own key and meaning; the command is typed into the pane's tag after a bare `\|` marker rather than into a popup | out of corpus |
+| `A-\|` | the same, and the output is DISCARDED — the text is not touched at all | helix `shell_pipe_to`. For a command run for its effect. Marker `\|-` | out of corpus |
+| `!` | run with NO stdin, insert the output BEFORE each selection | helix `shell_insert_output`. Runs ONCE and every cursor gets that one answer, as helix does — ten cursors and `date` give ten identical stamps. Marker `!` | out of corpus |
+| `A-!` | the same, appended AFTER each selection | helix `shell_append_output`. Marker `!+` | out of corpus |
 
 ## B. To implement