fix: preserve selection anchor for multi-Select range selection

[plore]

Closes #10147

Problem

Shift+click (and shift+ArrowUp/Down) range selection did not work in Select/Picker with selectionMode="multiple".
useSelectState exposes the selection as a plain Key[] and rebuilds an anchorless Selection every render, so SelectionManager.extendSelection never had an anchor and the range collapsed to just the clicked item.

Solution strategy

Keep the last Selection produced by the listbox and feed it back as selectedKeys while its membership still matches value, so anchorKey survives the round-trip.
Single-selection is untouched; external value changes (clear/reset/controlled) fall back to the plain array.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Open a Select/Picker with selectionMode="multiple", click an option, then shift+click another — the full range fills in.
Shift+clicking again from the same anchor re-extends/shrinks the range.

[LFDanLu]

Looks good to me, will see about getting another pair of eyes on this

[LFDanLu]

I was thinking maybe we could preserve the anchorKey and currentKey from the Selection instead and preserve them instead of walking and replicating the Selection, but I suppose we would still need to guard against if the set of selected keys changed outside of this flow still and thus walk the set anyways

[devongovett]

I think this technically breaks the rules of react because it reads from a ref during render.

[devongovett]

alternative suggestion: we could associate the extra info directly with the value via a WeakMap in onChange, and read it here.

let selectionMap = new WeakMap();

// in onChange
let value = [...keys];
selectionMap.set(value, keys);

// in render, when getting selectedKeys
let selectedKeys = selectionMap.get(displayValue) || convertValue(displayValue);

[snowystinger]

Did a little looking into it, the current implementation does indeed fail the compiler, though it isn't being caught in lint at the moment even when updated, not sure why. You can see more info here #10174