[lavrton]

What does this PR do and why?

Keeps a node's snapshot object the same across lazy instantiation.

Before a node is read, its snapshot computed returns the cached initial snapshot, and parents embed that object in their own snapshots. createObservableInstance then calls trackAndCompute(), which replaces it with a fresh, structurally identical object without notifying the parents. From then on a snapshot taken before the read never matches by identity, so applySnapshot re-applies every child of it (ArrayType.applySnapshot, ModelType.applySnapshot and areSame skip a child only when given the very object it holds). An undo manager that records getSnapshot(root) after a load pays O(all children) every time it undoes to that state: ~560 ms for 10,000 elements in Polotno, ~1 ms after this change.

The computed now treats the recompute during INITIALIZING as unchanged, so MobX keeps the previous object. Instantiation changes no data; afterCreate runs after that window and propagates as before.

Steps to validate locally

• bun test __tests__/core/snapshot-identity.test.ts — both tests fail on master.
• NODE_ENV=development bun test and MST_TESTING=1 NODE_ENV=production bun test are green.