Optimize state tracing of display state for nodes

[PimentelM]

We should come up with a way to make the displayState of nodes to only be sent over the wire when a change occurred.

We can design a way where we create special kind of object that will be used to track internal changes to the state of a node and only serialize to a defined value when a change occured.

Or we can add a boolean to nodes that resets at the begining of each tick, and a method to set that boolean to true. if that field is true the serializer will pick the displayState. if not it will omit it. Node implementations will need to manually call this method every time a mutation to information present in displayState is done.

This should be implemented in such a way that we maximize user experience.

[PimentelM]

Implementation Notes

Hot path: tree.ts:91-96 calls node.getDisplayState() for every ticked node unconditionally when stateTraceEnabled. ~16 node types implement it. Result serialized into TickTraceEvent.state and sent over wire even when unchanged.

Three approaches:

Option A — Dirty flag: Add _displayStateDirty to BTNode, markDisplayStateDirty() method. Only call getDisplayState() when dirty. Con: must touch all 16 implementations, error-prone for user nodes.

Option B — Previous-value diffing (recommended): BehaviourTree maintains Map<nodeId, SerializableState> of last-sent state. Compare via JSON.stringify equality. Zero node changes, works for user-defined DisplayState lambdas.

Option C — Proxy-based TrackedState: Most invasive, best DX for custom nodes.

Recommendation: Option B — zero breaking changes, works for all node types. Follow-up can add opt-in dirty flag (A) for hot-path nodes.

Open questions:

• Omitted state in TickTraceEvent means "use previous", or add stateUnchanged: true field?
• Time-based nodes (Timeout, Delay, Sleep) change every tick while running — exclude from dedup?
• Special treatment for DisplayState (user lambda) since library can't know when closure's return value changes?

[PimentelM]

Complexity: 5/9
Scope: 2/3 | Uncertainty: 2/3 | Dependencies: 1/3
~16 nodes implement getDisplayState; needs dirty flag in BTNode base + opt-in calls in each node + serializer/trace gating. All within core pkg, no external deps. Design choice (dirty flag vs change-tracking proxy) needs deciding but both approaches are well understood.