refactor: use Set instead of Array for O(1) listener operations in singleton-handler

[TrevorBurnham]

This PR replaces the Array with a Set in createSingletonHandler for managing listeners, improving the time complexity of listener removal from O(n) to O(1).

Motivation

The createSingletonHandler is used by useMutationSingleton, which powers useCurrentMode, useDensityMode, and useReducedMotion. These hooks observe document.body with { attributes: true, subtree: true }, meaning the MutationObserver fires on every attribute change anywhere in the DOM.

In applications with many Cloudscape components using these hooks, the listener collection can grow large. The previous implementation used:

listeners.splice(listeners.indexOf(listener), 1);

This is O(n) for each component unmount. With the Set-based approach:

listeners.delete(listener);

Removal is O(1).

Changes

• src/internal/singleton-handler/index.ts: Replace Array with Set for listener storage
• src/internal/singleton-handler/__tests__/create-singleton-handler.test.tsx: Add tests for listener order preservation and rapid mount/unmount cycles

Complexity comparison

Operation	Array (before)	Set (after)
Add listener	O(1) push()	O(1) add()
Remove listener	O(n) indexOf() + splice()	O(1) delete()
Check empty	O(1) length	O(1) size
Iterate	O(n)	O(n)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (c09d82c) to head (d613b87).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #187   +/-   ##
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  Coverage   99.00%   99.00%           
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  Files          42       42           
  Lines        1101     1101           
  Branches      280      296   +16     
=======================================
  Hits         1090     1090           
+ Misses         11       10    -1     
- Partials        0        1    +1     

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