feat: add R command

[MukundaKatta]

Summary

• add GNU sed's R command outside POSIX mode
• share one lazy reader per filename so repeated R commands consume one line at a time
• silently ignore missing files and EOF, matching GNU behavior
• cover repeated reads, missing/EOF behavior, and POSIX rejection

Fixes #394.

Tests

• cargo fmt --check
• cargo test read_one_line
• manual reproduction: cargo run --quiet -- -e "1R<file>" -e "2R<file>" <input>
• cargo test test_sed
• cargo test

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 65 / PASSED: 9 / FAILED: 46 / SKIPPED: 10
  Reference: TOTAL: 65 / PASSED: 8 / FAILED: 47 / SKIPPED: 10

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

Test improvements (1):
  + cmd-R

[codecov]

Codecov Report

❌ Patch coverage is 2.56410% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.66%. Comparing base (f083fa0) to head (23a0a1a).

Files with missing lines	Patch %	Lines
src/sed/compiler.rs	0.00%	19 Missing ⚠️
src/sed/processor.rs	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
- Coverage   82.20%   81.66%   -0.55%     
==========================================
  Files          13       13              
  Lines        5542     5579      +37     
  Branches      310      315       +5     
==========================================
  Hits         4556     4556              
- Misses        983     1020      +37     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	82.33% <2.56%> (-0.57%)	⬇️
ubuntu_latest	82.44% <2.56%> (-0.56%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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[codspeed-hq]

Merging this PR will improve performance by 2.75%

⚠️ 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
⚡	genome_subst	215.5 ms	209.7 ms	+2.75%

Tip

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

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_{Comparing MukundaKatta:codex/add-r-line-command (23a0a1a) with main (f083fa0)}