[Parity] Add declarative gradient stops and resource composition

[jonobr1]

Summary

Make gradients usable without constructing Two.Stop manually or round-tripping a callback ref through React state.

The current LinearGradient and RadialGradient wrappers accept a stops array, but there is no exported Stop component or hook and no declarative relationship between a gradient resource and a shape. The current README example supplies plain stop objects; Two.js expects Two.Stop instances and throws when it tries to bind those objects.

Desired capabilities

A final API may use components, hooks, resource props, or a combination, but it should support an ergonomic equivalent of:

<LinearGradient x1={0} y1={0} x2={1} y2={1}>
  <Stop offset={0} color="red" />
  <Stop offset={1} color="blue" />
</LinearGradient>

Consumers should also be able to assign the resulting resource to fill or stroke without a fragile callback-ref + state pattern.

Related work

• [Feature Request] Add Two.Texture #1 introduced the Texture component.
• Feature Request: Add useTexture hook for async texture loading with states #12 tracks asynchronous texture loading and caching.
• Shared ownership, replacement, and disposal semantics should align with the scene reconciliation work.

Acceptance criteria

• Export a typed declarative representation of Two.Stop.
• Support adding, removing, updating, and reordering stops through React.
• Provide an ergonomic, documented way to connect a gradient to fill or stroke.
• Support shared gradients without disposing a resource still used by another shape.
• Dispose renderer/GPU/SVG resources when the owning React resource unmounts.
• Work across SVG, Canvas, and WebGL renderers.
• Replace the invalid plain-object README example with an executable example.
• Add tests for stop updates, stop ordering, shared resources, replacement, and cleanup.

[jonobr1]

Foundation Update from #29 & #30

The lifecycle and property foundation for gradient resources is now in place via #36 (closing #29 and completing #30):

• LinearGradient and RadialGradient are refactored to use the unified useTwoObject hook (isSceneObject: false), providing discrete prop diffing and Strict Mode double-mount safety.
• Endpoint properties (left, right, center, focal) now support VectorProp (Two.Vector, { x, y }, [x, y]) and update without overwriting internal vector instances.
• Owned vs shared resource disposal semantics have been established and tested, unblocking declarative <Stop /> composition and resource binding.