tar: add --backup[=CONTROL] and --suffix options

[nnosal]

Summary

Implements --backup[=CONTROL] and --suffix=STRING extraction options as described in issue #250.

• --backup (no value) backs up existing files using the existing strategy (numbered if .~1~ present, otherwise simple).
• --backup=CONTROL accepts none/off, simple/never, numbered/t, existing/nil.
• --suffix=STRING overrides the default suffix (~) used by the simple backup strategy.

Backup logic lives in src/uu/tar/src/operations/extract.rs. The three helper functions (simple_backup_path, numbered_backup_path, backup_file) are pure and independently testable.

Test plan

• cargo test --all passes (113 tests including 9 new backup-specific unit tests)
• cargo fmt --all -- --check clean
• cargo clippy --workspace --all-targets -ptarapp -D warnings clean
• Unit tests cover all four BackupControl variants, custom suffix, no-op when no existing file, numbered increment, and existing fallback logic

Fixes #250

[codspeed-hq]

Merging this PR will degrade performance by 11.61%

⚠️ 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 regressed benchmark
✅ 6 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	build_cli_command	44.3 µs	50.1 µs	-11.61%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing nnosal:feat/backup (bb83a62) with main (9aa518b)}

[codecov]

Codecov Report

❌ Patch coverage is 93.80165% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.26%. Comparing base (791ae26) to head (bb83a62).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/tar/src/tar.rs	75.60%	10 Missing ⚠️
src/uu/tar/src/operations/extract.rs	85.71%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
+ Coverage   96.84%   97.26%   +0.41%     
==========================================
  Files          11       15       +4     
  Lines        1492     1976     +484     
  Branches       29       37       +8     
==========================================
+ Hits         1445     1922     +477     
- Misses         46       53       +7     
  Partials        1        1              

Flag	Coverage Δ
macos_latest	97.26% <93.80%> (+0.41%)	⬆️
ubuntu_latest	97.26% <93.80%> (+0.41%)	⬆️
windows_latest	0.00% <0.00%> (ø)

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