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 88.09524% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.90%. Comparing base (f1c5813) to head (49e0e12).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	6 Missing ⚠️
src/sed/compiler.rs	89.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
- Coverage   83.04%   82.90%   -0.14%     
==========================================
  Files          13       14       +1     
  Lines        7046     6949      -97     
  Branches      401      405       +4     
==========================================
- Hits         5851     5761      -90     
+ Misses       1192     1185       -7     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	84.02% <88.09%> (+0.35%)	⬆️
ubuntu_latest	84.24% <88.09%> (+0.36%)	⬆️
windows_latest	0.00% <0.00%> (ø)

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[codspeed-hq]

Merging this PR will improve performance by 3.17%

⚠️ 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
⚡	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.

---

_{Comparing Jorge-Polanco-Roque:feat/R-read-one-line (49e0e12) with main (77f442d)}

[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.

[Jorge-Polanco-Roque]

Added integration tests for R covering the cases that were missing: address-gated R that only advances when it runs, independent cursors across different files, and a final line without a trailing newline. While writing them I also caught and fixed a real divergence from GNU: multiple R on the same file now share one cursor instead of each restarting from line 1. Full suite and clippy are green.

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 65 / PASSED: 15 / FAILED: 42 / SKIPPED: 8
  Reference: TOTAL: 65 / PASSED: 14 / FAILED: 43 / SKIPPED: 8

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

Test improvements (1):
  + cmd-R