fast_io.rs: use sendfile or splice if copy_file_range is not supported

[Franklin-Qi]

• Use splice and attempt to expand the pipe size when necessary.
• Modify the reliable_copy_file_range() function to sequentially attempt copy_file_range → splice → sendfile → write.
• Add Linux-based unit tests.

Closes: #443

[codecov]

Codecov Report

❌ Patch coverage is 72.41379% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.85%. Comparing base (a9a332a) to head (581e725).

Files with missing lines	Patch %	Lines
src/sed/fast_io.rs	72.41%	56 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
- Coverage   82.18%   81.85%   -0.33%     
==========================================
  Files          13       13              
  Lines        5551     5743     +192     
  Branches      312      323      +11     
==========================================
+ Hits         4562     4701     +139     
- Misses        986     1039      +53     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	82.86% <ø> (ø)
ubuntu_latest	82.61% <72.41%> (-0.36%)	⬇️
windows_latest	0.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.