pipe: a filter that fails says so, and `| head -1` stops failing

Two things made the selection pipe feel like it had never worked. It runs —
test/snapshots/pipe.snap drives the real binary through a pty and filters
`alpha beta` to `ALPHA BETA` — but it had no way to tell you when it did not,
and one of its failure conditions was not a failure at all.

IT NOW SAYS WHY. `runOne` read the command's stderr into memory and freed it
two lines later, unread; every caller answered a failed filter with a bare
`return`; `pipeResponse` had eight more silent exits under that. So `| trr`
(a typo), `| grep nomatch` (exit 1), `| jq .` on bad JSON — all did nothing,
said nothing, and left the text alone with no way to find out why. The runner
carries a `Failure` home instead: which selection, what became of the command,
and its own stderr. The core turns that into an `+Errors` buffer — acme's name
for output that came from the program rather than from a word anybody clicked:

    | trr

    exit status 127
    sh: line 1: trr: command not found

An output buffer rather than the message row because the useful half of a
shell failure is the text the shell wrote, and a 256-byte row would keep the
label and throw away the reason. Focus stays with the file: `openRead` moves
`p.active` to what it opens, which is right for a Grep you asked to read and
wrong for a report you did not — you want to fix the command and press `|`
again. A host with no `pull_pipe` at all (the detached daemon, the browser,
the board) now says that too, instead of answering failure into the void.

`| head -1` NOW WORKS. `writer_context.ok` was part of the success condition,
so a command that stopped reading its stdin failed the filter even though it
had done exactly its job: `head` takes the line it wants and closes the pipe,
the write gets EPIPE, and a selection bigger than the 64 KiB pipe buffer was
enough to trigger it. helix joins its input task and ignores the result for
this reason; the exit status is the whole verdict. Also reported rather than
swallowed: the ten-second timeout, the output ceilings, and a file edited
while the filter ran — one keystroke during a slow command used to discard the
result in a way indistinguishable from the filter doing nothing.

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

0 files changed, 0 insertions, 0 deletions