sed: write appended text, R text and e output as GNU sed does

[DePasqualeOrg]

Please ignore this PR until it is marked as ready for review

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This PR builds on #592, which fixes n; only its last commit is new. It now also includes the change from #590, because each needed the other.

When the last line of input has no final newline and the cycle ends with text queued by a, r or R, GNU sed first writes the missing newline and then the queued text, even when an r file is missing or empty. uutils sed joins them instead: printf a | sed 'r f' prints a immediately followed by the contents of f. This PR writes the missing newline before any text queued for the end of the cycle.

GNU sed copies the line that R reads, and the output of the e command form, exactly as they are. uutils sed treats them as lines and adds a newline when they lack one. With a file f that contains x and no newline, printf 'a\nb\n' | sed 'R f' prints x and b on separate lines, while GNU sed prints xb. Likewise, printf 'a\n' | sed '1e printf hi' prints hi on a line of its own, while GNU sed prints hia. This PR adds OutputBuffer::write_raw, which writes a newline that the previous output line lacks and then the bytes unchanged, and uses it for this text.

Q now discards text queued by a, r and R, as GNU sed does. Before, printf 'x\n' | sed -e 'a A' -e Q printed A. q still writes queued text. Finally, l now prints $ for an empty pattern space whose line lacked a newline.

codecov/patch and codecov/project fail only because CI does not count the coverage of tests that run the sed binary; they should pass after #591 is merged.

[codecov]

Codecov Report

❌ Patch coverage is 27.58621% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.79%. Comparing base (1113479) to head (ae5f8dc).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	12.50%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #589      +/-   ##
==========================================
- Coverage   83.86%   83.79%   -0.08%     
==========================================
  Files          14       14              
  Lines        7203     7219      +16     
  Branches      424      428       +4     
==========================================
+ Hits         6041     6049       +8     
- Misses       1157     1165       +8     
  Partials        5        5              

Flag	Coverage Δ
macos_latest	84.90% <27.58%> (-0.09%)	⬇️
ubuntu_latest	85.09% <27.58%> (-0.08%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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[codspeed]

Merging this PR will improve performance by 36.51%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 10 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	access_log_append	1.5 s	1.1 s	+36.51%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing DePasqualeOrg:fix-r-separator-and-q-appends (ae5f8dc) with main (1113479)}

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 65 / PASSED: 18 / FAILED: 39 / SKIPPED: 8
  Reference: TOTAL: 65 / PASSED: 17 / FAILED: 40 / SKIPPED: 8

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + distrib