feat(partial-vec): add methods for getting and mutating single items

[vapourismo]

What

Why

How

Manually Testing

make all

Regressions

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Explain changes to regression test captures when applicable.
• Write commit messages in agreement with our guidelines.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.08%. Comparing base (214b4ab) to head (6ac08b9).

Files with missing lines	Patch %	Lines
data/src/partial_vec.rs	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #786      +/-   ##
==========================================
- Coverage   91.10%   91.08%   -0.02%     
==========================================
  Files         110      110              
  Lines       20913    20931      +18     
  Branches    20913    20931      +18     
==========================================
+ Hits        19052    19066      +14     
- Misses       1488     1491       +3     
- Partials      373      374       +1     

☔ View full report in Codecov by Sentry.
📢 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.