Add/remove mesh instance

[LeXXik]

Fixes #6680

Adds new API:

const mesh = Mesh.fromGeometry(app.graphicsDevice, new BoxGeometry());
const instance = new MeshInstance(mesh, new StandardMaterial());

entity.render.addMeshInstance(instance);
entity.render.removeMeshInstance(instance);

Tests included.

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.

[Maksims]

This is very useful! We had cases where we needed to do that, we had to slice the meshInstances array, then modify it and assign again, but that leads to unnecessary re-initializations and layers stuff, is just not efficient to do at runtime as it could be.
In this PR there is no unnecessary work, which is great.

[Maksims]

We need this!

[AlexAPPi]

@mvaligursky Could you please review and approve this PR? It's sorely needed.

[mvaligursky]

It's definitely on my list, sorry this is taking a while.

[Copilot]

Pull request overview

Adds a public API to dynamically add/remove MeshInstance objects on RenderComponent, addressing the need for incremental procedural mesh management without resetting the whole meshInstances array.

Changes:

• Introduces RenderComponent#addMeshInstance() / RenderComponent#removeMeshInstance() and refactors internal per-mesh instance update logic.
• Extends Layer with single-instance add/remove helpers used by the component.
• Adds unit tests covering add/remove behavior and layer membership updates.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
test/framework/components/render/component.test.mjs	Adds tests for the new RenderComponent mesh instance add/remove API and layer integration.
src/scene/layer.js	Refactors mesh/shadow caster add/remove to support single-instance operations and shared shader-variant clearing.
src/framework/components/render/component.js	Adds the new public API, refactors mesh instance property updates, and updates layer add/remove helpers.

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[Copilot]

Pull request overview

This PR adds a public API on RenderComponent to dynamically add and remove individual MeshInstances without needing to replace the whole meshInstances array, addressing the workflow described in #6680.

Changes:

• Added RenderComponent#addMeshInstance and RenderComponent#removeMeshInstance (with optional destruction behavior).
• Extended Layer with single-instance add/remove helpers and refactored multi-instance paths to reuse them.
• Added unit tests covering add/remove behavior and layer membership updates.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/framework/components/render/component.js	Implements the new RenderComponent API and refactors shared mesh-instance update/cleanup logic.
src/scene/layer.js	Adds single-instance layer/shadow-caster helpers and consolidates shader-variant clearing.
test/framework/components/render/component.test.mjs	Adds tests validating add/remove behavior and that layers are updated accordingly.

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