feat(interpreter): stream yes and seq into pipelines with SIGPIPE

[chaliy]

Requested by Михайло · project thread

What changed

yes and seq stream into pipelines. As a producer stage they now run concurrently, like shell-code stages already did. They write 4 KiB chunks straight into the stage pipe through a new internal Context::stdout_stream(). When the reader is gone they stop with 141 (SIGPIPE).

The pipe goes only to the stage's own unredirected simple command, matched by address. Commands run from its arguments, a yes() {...} function and yes > file keep the buffered path.

Why

This is step 2 of the roadmap (streaming builtin I/O). The roadmap names seq 1e6 | head -1 and yes | head as the motivating cases. Both used to run to the output cap and print a truncation notice.

Before

$ seq 1000000 | head -1; echo "${PIPESTATUS[*]}"
1
seq: line limit (100000) exceeded; output truncated
1 0

After

1
141 0

yes | head -2 gives 141 0 and set -o pipefail; yes | head -1 exits 141, both as in bash. The new spec pipe-generators.test.sh matches real bash. The L-PIPE-001 evidence now uses cat big | head -1, which still runs to completion.

Risk

Low to medium.

• It reuses the existing bounded pipe and its nesting cap (TM-DOS-124).
• The output caps still apply when the reader takes everything (yes | wc -l).
• Each chunk charges budget work.

Checklist

• Failing spec first, compared with bash
• lib + integration tests, clippy, fmt
• knowledge (parallel-execution, limitations, log)

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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