fix(labelmap): use content-based comparison for segmentsHidden Set

[pedrokohler]

Context

_needsTransferFunctionUpdate() in labelmapDisplay.ts compares
oldSegmentsHidden !== segmentsHidden using reference equality. Because
segmentsHidden is a new Set instance on every render call, this comparison
always returns true — even when the contents are identical. This forces
forceOpacityUpdate = true unconditionally, triggering unnecessary .slice() and
new Set() allocations on every render cycle for every segmentation.

With 13 segmentations across ~180 render calls per scroll, this produces hundreds
of redundant transfer-function updates per interaction.

Changes & Results

Replace the reference comparison with a content-based comparison:

1. Check size equality first (fast path).
2. If sizes match, check [...segmentsHidden].some(idx => !oldSegmentsHidden.has(idx)).

Before: transfer function updated on every render (~180 times per scroll).
After: transfer function updated only when segments are actually hidden/shown (~15 times total in a typical session).

Testing

1. Open a StackViewport with multiple labelmap segmentations.
2. Scroll through slices without toggling segment visibility.
3. Place a breakpoint or log in _needsTransferFunctionUpdate — forceOpacityUpdate
   should be false when no visibility changes occurred.

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Made with Cursor

[pedrokohler]

@sedghi this is a performance optimization, it reduces allocation churn / CPU overhead during scrolling.

[sedghi]

Thanks, i let QA test it

[pedrokohler]

@sedghi it's been almost 2 months since QA was asked to review. Could we merge?

[sedghi]

@james-hanks