fix: Timeline filters don't apply to tooltips

[traplol]

Summary

• Apply category filters to timeline hit-testing so that hidden events no longer show tooltips or respond to clicks
• Extract select_best_candidate() as a public static method on TimelineView for testability
• Add 7 unit tests covering category filtering, end-event skipping, depth matching, time range, and empty candidates

Details

When a category filter was turned off, the rendering loop correctly skipped hidden events, but the hit_test() function still matched them — causing phantom tooltips to appear on hover. The fix adds the same hidden_cats check to the candidate selection logic inside hit_test().

To make this testable without ImGui state, the inner candidate filtering loop was extracted into TimelineView::select_best_candidate(), a pure function that takes candidates, events, hidden categories, depth, time, and tolerance, and returns the best matching event index.

Closes #67

Test plan

• HiddenCategoryFilteredOut — hidden category event skipped, visible one selected
• AllCandidatesHiddenReturnsNone — returns -1 when all candidates hidden
• NoCategoryFilterSelectsShortest — without filters, shortest duration wins
• EndEventsSkipped — end events always excluded
• WrongDepthSkipped — depth mismatch excluded
• OutOfTimeRangeSkipped — out-of-range excluded
• EmptyCandidatesReturnsNone — empty input returns -1

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