Implement GNU R (read one line from file) command

[Jorge-Polanco-Roque]

Closes #394.

Implements the GNU R command, the line-oriented counterpart to r (mirroring how W relates to w, added in #531).

R filename queues the next successive line of filename to be inserted into the output stream at the end of the current cycle. Each cycle consumes one more line; once the file is exhausted no line is queued.

Behavior

$ printf 'a\nb\nc\nd\n' | sed -e 'R lines.txt'   # lines.txt = x1,x2,x3
a
x1
b
x2
c
x3
d

• The file is opened lazily on first use and read one line at a time (new NamedReader, analogous to NamedWriter).
• An unreadable or exhausted file yields no further lines — no error is raised, matching GNU sed.
• Reserved to non-POSIX mode and rejected under --sandbox, exactly like r/w/W.

Tests

• Unit tests for NamedReader (successive lines, final line without newline, missing file).
• Integration tests: successive-line behavior, silent missing file, --posix rejection, --sandbox rejection.

cargo fmt --all -- --check, cargo test --all, and cargo clippy --all-targets --workspace -psed -- -D warnings all pass locally.

[codecov]

Codecov Report

❌ Patch coverage is 87.65432% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.09%. Comparing base (f1c5813) to head (a3e6cc3).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	6 Missing ⚠️
src/sed/compiler.rs	88.57%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
+ Coverage   83.04%   83.09%   +0.05%     
==========================================
  Files          13       14       +1     
  Lines        7046     7127      +81     
  Branches      401      405       +4     
==========================================
+ Hits         5851     5922      +71     
- Misses       1192     1202      +10     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	83.71% <87.65%> (+0.04%)	⬆️
ubuntu_latest	83.92% <87.65%> (+0.04%)	⬆️
windows_latest	0.00% <0.00%> (ø)

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[codspeed-hq]

Merging this PR will improve performance by 3.17%

⚡ 1 improved benchmark
✅ 10 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	access_log_translit	1 s	1 s	+3.17%

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 Jorge-Polanco-Roque:feat/R-read-one-line (a3e6cc3) with main (f1c5813)}

[sylvestre]

please add tests to improve the coverage. some tests are missing to reach important cases

[Jorge-Polanco-Roque]

Thanks — added tests to cover the missing cases: unit tests for compile_read_line_command (sandbox rejection and that it installs a NamedReader), plus integration tests for R appending under -n (suppressed auto-print) and for a source file shorter than the input. The R compile and execute paths are now exercised directly.