implement xargs -P using rayon and a spin lock

[codspeed-hq]

Add CodSpeed benchmarks for the parallel xargs -P path

This PR introduces parallel execution for xargs -P (via rayon), but the existing benchmark suite (benches/xargs_bench.rs) only exercised the serial path — so CodSpeed reported the new code as untouched.

This change extends benches/xargs_bench.rs with three benchmarks that drive the new rayon-based dispatch in process_input:

• parallel_p2 — -n 10 -P 2: a low degree of parallelism. This is the spin-lock worst case noted earlier in the review (high contention between two workers).
• parallel_p8 — -n 10 -P 8: higher parallelism, exercising the scheduler under more contention.
• parallel_p8_batched_n — -n 50 -P 8: fewer, larger batches so the balance shifts back toward argument assembly while still going through the parallel bridge.

Implementation notes

• The parallel benchmarks use a smaller dedicated corpus (400 tokens) than the serial ones (4000). Each batch spawns a real true child, so a smaller corpus keeps the number of spawns reasonable under the CodSpeed simulation instrument while still producing enough batches to feed the parallel bridge.
• The serial benchmarks (split_whitespace, split_null, batched_n, batched_size) are unchanged.
• Measurements are dominated by xargs's own work (reading, splitting, batching, and parallel dispatch); child-process spawns are not traced under simulation.

All benchmarks build and run successfully under codspeed run --mode simulation.

[codecov]

Codecov Report

❌ Patch coverage is 93.38843% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.74%. Comparing base (f38dbe5) to head (017bea8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/xargs/mod.rs	93.38%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #794      +/-   ##
==========================================
+ Coverage   91.89%   92.74%   +0.85%     
==========================================
  Files          35       35              
  Lines        7217     7751     +534     
  Branches      376      461      +85     
==========================================
+ Hits         6632     7189     +557     
+ Misses        442      419      -23     
  Partials      143      143              

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[github-actions]

Commit 017bea8 has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 413 / FAILED: 81 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: -4
  FAILED: +4

New test failures (4):
  - idef-s26-0
  - s14
  - s14-0
  - s42

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6

New test failures (1):
  - gnu/files0_from_ok

Test improvements (1):
  + gnu/okdir_path_empty

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks
🆕 3 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	parallel_p2	N/A	62.9 ms	N/A
🆕	parallel_p8	N/A	9.4 ms	N/A
🆕	parallel_p8_batched_n	N/A	2.5 ms	N/A

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_{Comparing push-zlxwkxlkpsls (017bea8) with main (3f9926c)}