Fix LoadingList::selected() to return item reference

[ryanoneill]

Summary

• Rename selected() → selected_index() (returns Option<usize>)
• Add new selected() → Option<&LoadingListItem<T>> consistent with Tabs, RadioGroup pattern
• Make selected_item() an alias for selected()
• Update all ~27 test call sites from selected() to selected_index()
• Add test_selected_returns_item test

Context

Every other Envision component's selected() returns an item reference (Option<&T>). LoadingList was the outlier, returning Option<usize>. The index-returning method is still available as selected_index().

Test plan

• All 75 LoadingList unit tests pass
• 3 doc tests pass (including new doc test on selected())
• cargo clippy --all-targets --all-features -- -D warnings clean

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.64%. Comparing base (e096986) to head (3eaee83).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #114   +/-   ##
=======================================
  Coverage   92.64%   92.64%           
=======================================
  Files         100      100           
  Lines        9545     9547    +2     
=======================================
+ Hits         8843     8845    +2     
  Misses        702      702           

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