sed: write the newline a file's last line lacks before the next file's output, as GNU sed does

[DePasqualeOrg]

When an input file's last line has no final newline and another file follows, GNU sed writes the missing newline before the next output. uutils sed drops it, so the last line of one file runs into the first output of the next: with printf a > 1 and printf 'b\n' > 2, sed '' 1 2 prints ab instead of a and b on separate lines. The same happens in w and W files.

This PR carries the deferred newline from one file's output buffer to the next, and gives NamedWriter the same deferred newline for w, W and s///w files. W now passes its first line without the newline, so that the writer knows whether the line ended. With -i, each file keeps its own output, so nothing is carried between files.

0r now writes its file before a newline carried over from the previous file, which then comes before that file's first line, as in GNU sed. copy_file gains a variant that leaves the deferred newline in place for this.

-z is still ignored, so with -z the files also get a newline between them, as they do without it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.98%. Comparing base (96afb29) to head (2a6a4c7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
+ Coverage   94.85%   94.98%   +0.13%     
==========================================
  Files          14       14              
  Lines        7230     7265      +35     
  Branches      432      432              
==========================================
+ Hits         6858     6901      +43     
+ Misses        371      363       -8     
  Partials        1        1              

Flag	Coverage Δ
macos_latest	96.52% <100.00%> (+0.13%)	⬆️
ubuntu_latest	96.46% <100.00%> (+0.12%)	⬆️
windows_latest	0.00% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed]

Merging this PR will regress 1 benchmark

⚠️ 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
❌ 1 regressed benchmark
✅ 9 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	access_log_append	1 s	1 s	-2.01%
⚡	access_log_translit	1,019.5 ms	989.7 ms	+3.02%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing DePasqualeOrg:fix-separator-between-files (2a6a4c7) with main (96afb29)}