[Parity] Add a Primitive escape hatch for existing and custom Two.js objects

[jonobr1]

Summary

Add a generic React component for attaching an existing Two.js object to the declarative scene graph.

A primitive escape hatch makes custom Two.Shape subclasses, extras, clones, and objects restored with fromObject(...) usable without waiting for a bespoke wrapper or imperatively calling parent.add(...).

Proposed API sketch

const object = useMemo(() => Two.Group.fromObject(snapshot), [snapshot]);

<Primitive object={object} />

The exact name and ownership API are open for design.

Related work

• Feature Request: Add Arc component for open arc shapes #10 should still provide a first-class typed Arc component.
• This should use the same ordering, reparenting, event registration, and cleanup semantics as built-in components.

Acceptance criteria

• Attach a supplied Two.js Shape or Group to the current React parent.
• Remove and reparent it correctly as React ownership changes.
• Preserve React sibling ordering.
• Forward a ref to the supplied object.
• Define whether the wrapper owns/disposes the object, with an explicit option if both owned and borrowed objects are supported.
• Define how prop overrides and event handlers apply to primitives.
• Allow React children when the supplied object is a Group, or clearly document why this is unsupported.
• Support an Arc-extra or custom Shape example.
• Support a Group.fromObject(...) scene restoration example.
• Add lifecycle, ordering, borrowed-resource, and custom-object tests.

[jonobr1]

Foundation Update from #29

The lifecycle and reconciliation primitives required for <Primitive object={...} /> are now available via #36 (closing #29):

• useTwoObject centralizes scene-graph attachment, dynamic reparenting, JSX sibling order registration (registerChildOrder), in-place replacement, ref forwarding, and event registration.
• An implementation of <Primitive object={object} /> can directly wrap useTwoObject(props, forwardedRef, { factory: () => object, isSceneObject: true }), instantly gaining authoritative scene graph ordering, reparenting, and Strict Mode double-mount safety.