chore: useIconChecker should return a stable predicate via useMemo

[jacuzzicoding]

Summary

useIconChecker in src/components/itemIconUtils.ts is documented as returning a "stable predicate" but allocates a fresh closure on every render. Stable-by-name, not stable-by-reference. Doesn't break correctness today, but any consumer that relies on === referential stability (memoized children, dependency arrays) sees a new function each render.

Fix sketch

Wrap the returned closure in useMemo keyed on state.manifest:

const checker = useMemo(
  () => (category: CategoryId, id: string) => {
    if (state.kind !== 'present') return false;
    return Boolean(resolveIconUrl(state.manifest, category, id));
  },
  [state.manifest, state.kind]
);

Or relax the docstring to "logically stable" if we don't actually need referential stability. Cheap to fix.

Acceptance criteria

• useIconChecker's returned function is referentially stable across renders when the manifest hasn't changed.
• Existing tests stay green; add one assertion that checker from two consecutive renders is === when the manifest is unchanged.

References

• Surfaced by Copilot review on PR #94, comment r3188656851.