Enable cursor support

[dubstar-04]

Complementary PR dubstar-04/Design-Core#260

fixes #10

[Copilot]

Pull request overview

Adds UI-level cursor support to the canvas component so the core can drive cursor changes (aimed at improving CAD-style interaction per Issue #10).

Changes:

• Register a cursor-change callback with core.canvas on mount and when core changes.
• Add onCursorChange to map cursor “states” to CSS cursor values.
• Unregister the cursor callback on unmount.

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[Copilot]

When core changes, the previous prevProps.core.canvas still retains the old cursor callback. That can keep this component alive and can also trigger onCursorChange after the canvas is no longer the active core (or after unmount), leading to errors. Clear the cursor callback on prevProps.core.canvas before registering the new one (mirroring cleanup done in componentWillUnmount).

Suggested change

	if (prevProps.core !== this.props.core) {
	if (prevProps.core !== this.props.core) {
	prevProps.core.canvas.setCursorCallbackFunction(undefined);

[Copilot]

this.onCursorChange.bind(this) creates a new function each time componentDidMount/componentDidUpdate runs. Consider binding once (e.g., this.boundOnCursorChange = this.onCursorChange.bind(this) in the constructor) and reusing that reference when registering/unregistering the callback; this avoids repeated allocations and makes cleanup reliable if the core API expects the same function reference.

[Copilot]

onCursorChange assumes this.canvasRef.current is always available. If a cursor update arrives after unmount (or while swapping cores), this will throw. Add a quick guard (e.g., return early when the ref is missing) to make the callback resilient.

Suggested change

	this.canvasRef.current.style.cursor = cursors[state] ?? 'crosshair';
	const canvas = this.canvasRef.current;
	if (!canvas) {
	return;
	}
	canvas.style.cursor = cursors[state] ?? 'crosshair';