Referential stability for observer result objects

[zeorin]

🎯 Changes

• Add result object property change notification to mutation observers. By default, property access is tracked and used to determine which properties to notify for.
• Compare result objects to most recently returned results, if they are shallowly equal, return a reference to the previous object rather than a new object that is equal by value but not by reference.
• When returning a proxy object for property access tracking, compare the previously tracked result and the new one only by the tracked properties, and if none of those have changed, return the previous reference.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr. (Didn't work for me, but npx nx run @tanstack/react-query:test:lib passes).

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

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Latest commit: 0de67d4

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[themavik]

QueryObserver.trackResult / MutationObserver trackResult caching with WeakMap + shallowEqualObjects on #updateResult should cut a lot of proxy churn. nit: render-optimizations.md now says hook return objects are stable unless RQ triggers a re-render — worth one clause that option/key changes still produce new result objects when underlying data moves, so readers do not read stable as frozen forever.