tar: add explicit zstd compression support

[kevinburke]

Add an explicit --zstd flag for create, list, and extract so .tar.zst archives work end to end.

Keep the scope narrow by requiring the flag instead of trying to autodetect compression. Add CLI coverage and end-to-end tests for creating, listing, and extracting zstd-compressed archives.

[codecov]

Codecov Report

❌ Patch coverage is 96.26667% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.50%. Comparing base (2c7a5e6) to head (eea4c22).
⚠️ Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/tar/src/operations/compression.rs	84.94%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #154       +/-   ##
===========================================
+ Coverage   60.83%   96.50%   +35.66%     
===========================================
  Files           7       11        +4     
  Lines         789     1288      +499     
  Branches       24       25        +1     
===========================================
+ Hits          480     1243      +763     
+ Misses        309       44      -265     
- Partials        0        1        +1     

Flag	Coverage Δ
macos_latest	96.50% <96.26%> (+35.66%)	⬆️
ubuntu_latest	96.50% <96.26%> (+35.66%)	⬆️
windows_latest	0.00% <0.00%> (ø)

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[kaladron]

Hi! I'm in the middle of implementing "-f -", and this patch as it's structured will make that work difficult. If you don't mind, I'd like to hold this patch until that lands and I'll rework it. (I also have #149 that centralises the error handling)

[kevinburke]

no problem - can you ping me when that's complete?

[kaladron]

Will do! I'm blocked on review of mine, and then I can review yours =)

[kevinburke]

Hello! I think this is ready for another look