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

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

• #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.

Start by tracing the existing built-in component lifecycle and the proposed API, focusing on ordering, reparenting, events, cleanup, and ref behavior. Done means the ownership and prop semantics are defined, custom and restored objects work, and the lifecycle, ordering, borrowed-resource, and custom-object tests described in the issue pass.