Improve GNU sed testsuite compatibility

[kevinburkesegment]

• Fix l command default width from 60 to 70 (matching GNU sed)
• Wire -l flag and COLS env var to l command width
• Reject l<N> (GNU extension) in --posix mode
• Accept } and # as valid command terminators (fixes command-endings)
• Accept } and # as valid substitute flag terminators
• Eat spaces before command endings (e.g. y/1/a/ ;d)
• Scope #n silent mode to first script source only (fixes comment-n)
• Drop backslash for unknown escape sequences in s/// replacement (GNU behavior)
• Match GNU sed error message format: -e expression #N, char C: for string scripts, file:line: for file scripts
• Add get_source_index() to ScriptLineProvider
• Add test-results.txt to .gitignore

GNU testsuite results: 7 → 9 passing (command-endings, comment-n now pass).

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 65 / PASSED: 9 / FAILED: 46 / SKIPPED: 10
  Reference: TOTAL: 65 / PASSED: 7 / FAILED: 48 / SKIPPED: 10

Changes from main branch:
  TOTAL: +0
  PASSED: +2
  FAILED: -2

Test improvements (2):
  + command-endings
  + comment-n

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 65 / PASSED: 9 / FAILED: 46 / SKIPPED: 10
  Reference: TOTAL: 65 / PASSED: 7 / FAILED: 48 / SKIPPED: 10

Changes from main branch:
  TOTAL: +0
  PASSED: +2
  FAILED: -2

Test improvements (2):
  + command-endings
  + comment-n

[codecov]

Codecov Report

❌ Patch coverage is 71.18644% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.92%. Comparing base (5e9b821) to head (e391df1).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sed/compiler.rs	65.62%	11 Missing ⚠️
src/sed/error_handling.rs	66.66%	5 Missing ⚠️
src/sed/script_line_provider.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #345      +/-   ##
==========================================
- Coverage   82.16%   81.92%   -0.25%     
==========================================
  Files          13       13              
  Lines        5444     5454      +10     
  Branches      304      312       +8     
==========================================
- Hits         4473     4468       -5     
- Misses        969      984      +15     
  Partials        2        2              

Flag	Coverage Δ
macos_latest	82.36% <71.18%> (-0.25%)	⬇️
ubuntu_latest	82.47% <71.18%> (-0.25%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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[sylvestre]

Sorry but only one commit implementating all the changes isn't the best practice. In a perfect world, one pr changes or at least one commit per fix in this pr

[sylvestre]

For example, it makes it hard to know what is cover by an existing test or not