fix(core): skip full-tree serialization when onNodesChange is not provided

[hnldlsjzt]

Problem

<Editor> unconditionally subscribes with a collector that calls query.serialize() — a JSON.stringify of the entire node tree — on every store change, solely to detect whether the onNodesChange callback should fire:

https://github.com/prevwong/craft.js/blob/master/packages/core/src/editor/Editor.tsx#L94-L103

Since onNodesChange defaults to a no-op, consumers that never pass it still pay this serialization cost on every state change.

Real-world impact

We render read-only pages (enabled: false) that receive frequent runtime prop updates over WebSocket (industrial monitoring dashboards running 24/7). On a page with ~500 nodes (~450 KB serialized JSON), CPU profiling showed:

• this serialization was the single largest JS cost (~20% of busy time);
• it was a major source of GC pressure — a ~450 KB string allocated and immediately discarded on every incoming data update.

With this fix applied (verified via pnpm patch in production), overall busy time on a 20-second profile dropped from 11.8% to 5.5%, with the serialize cost going to zero.

Fix

Only set up the subscription when the consumer explicitly provides onNodesChange. Consumers that pass the callback (i.e. editors) are unaffected — the subscription is created exactly as before. The guard also protects against context being null, consistent with the other effects in the component.

No API changes, no behavior changes for existing onNodesChange users.

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