fmt: fix fmt /dev/zero panic

[mattsu2020]

Summary

Prevent unbounded line buffering in fmt by introducing a capped, incremental line reader.
Split oversized paragraphs by byte/line thresholds to keep memory bounded on infinite input.

related
#10580

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/basenc/bounded-memory is now passing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/pr/bounded-memory. tests/pr/bounded-memory is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/factor/t33 is no longer failing!
Congrats! The gnu test tests/factor/t34 is no longer failing!
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will degrade performance by 3.68%

❌ 1 regressed benchmark
✅ 293 untouched benchmarks
⏩ 42 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	shuf_input_range[1000000]	84.8 ms	88 ms	-3.68%

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_{Comparing mattsu2020:fmt_devzero (78e0898) with main (6100aee)}

Footnotes

1. 42 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/factor/t10. tests/factor/t10 is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/factor/t33. tests/factor/t33 is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/factor/t34. tests/factor/t34 is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/factor/t28 is no longer failing!
Congrats! The gnu test tests/cut/cut-huge-range is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout-group. tests/timeout/timeout-group is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/date-locale-hour. tests/date/date-locale-hour is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/cut/bounded-memory is no longer failing!
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/timeout/timeout-group is no longer failing!
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!