sed: continue with the next command after n instead of starting a new cycle

[DePasqualeOrg]

Claude Code and Codex flagged this gap during my work with this project. The changes here have gone through extensive automated review. I'm happy to make changes as needed or close this PR and maintain it as a private patch if it doesn't meet your standards.

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Fixes #568.

GNU sed's n prints the pattern space unless -n is given, writes any text queued by a, r or R, replaces the pattern space with the next line of input, and continues with the command after n. In uutils sed, n ends the cycle instead, so the next line starts the script from the top. For example, printf '1\n2\n3\n4\n' | sed 'n;d' prints all four lines, while GNU sed prints 1 and 3.

This PR handles n the way N already works: it records the command to continue from, and when the next line has been read, processing resumes there with that line as the pattern space. If there is no next line, sed stops without running the remaining commands, as GNU sed does. Without -s, n reads the first line of the next file; with -s or -i, each file starts at the top of the script.

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.

[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

[codecov]

Codecov Report

❌ Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.76%. Comparing base (1113479) to head (a634674).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #592      +/-   ##
==========================================
- Coverage   83.86%   83.76%   -0.11%     
==========================================
  Files          14       14              
  Lines        7203     7212       +9     
  Branches      424      428       +4     
==========================================
  Hits         6041     6041              
- Misses       1157     1166       +9     
  Partials        5        5              

Flag	Coverage Δ
macos_latest	84.87% <0.00%> (-0.12%)	⬇️
ubuntu_latest	85.07% <0.00%> (-0.11%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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