[Parity] Add declarative gradient stops and resource composition #31

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.