Remove old per-item rendering pipeline and consolidate into GPU-driven path

[Copilot]

The GPU-driven renderer was still performing a per-item alloc_packet call for every VItem on each frame, and the old CPU-driven per-item rendering pipeline was never removed when the merged (GPU-driven) rendering path was introduced.

Changes

• Removed the per-VItem pool.alloc_packet loop in render_store_with_pool (packages/ranim-render/src/lib.rs)
• Removed the entire old per-item rendering pipeline:
  • Graph nodes: old VItemComputeNode, VItemDepthNode, VItemColorNode (per-item path)
  • Pipelines: old VItemColorPipeline, VItemDepthPipeline, VItemComputePipeline (per-item path) and associated bind groups
  • Shaders: old vitem.wgsl, vitem_compute.wgsl (per-item path)
  • VItemRenderInstance, PlaneUniform, and impl Primitive for VItem from primitives/vitem.rs
• Renamed the GPU-driven (Merged*) nodes and pipelines to drop the Merged prefix:
  • MergedVItemComputeNode → VItemComputeNode
  • MergedVItemDepthNode → VItemDepthNode
  • MergedVItemColorNode → VItemColorNode
  • MergedVItemComputePipeline → VItemComputePipeline
  • MergedVItemColorPipeline → VItemColorPipeline
  • MergedVItemDepthPipeline → VItemDepthPipeline
  • merged_vitem.wgsl → vitem.wgsl, merged_vitem_compute.wgsl → vitem_compute.wgsl
• Renamed build_merged_render_graph() → build_render_graph() and removed the deprecated old build_render_graph() in lib.rs

The GPU-driven path (now the only path) reads exclusively from MergedVItemBuffer, issuing a single batched draw call for all items rather than allocating and dispatching per-item GPU resources.

Original prompt

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This section details on the original issue you should resolve

<issue_title>GPU-driven rendering still do per-item alloc</issue_title>
<issue_description>related: #139 #138

My fault, forgot to remove.

ranim/packages/ranim-render/src/lib.rs

Lines 226 to 232 in 9e2b612

	// Per-VItem packets (old path nodes query these; merged nodes ignore them)
	self.packets.extend(
	store
	.vitems
	.iter()
	.map(|(_id, data)| pool.alloc_packet(ctx, data)),
	);

</issue_description>

<agent_instructions>Fix it</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes GPU-driven rendering still do per-item alloc #140

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[AzurIce]

Can completely remove the old per-item pipeline, and only keep the Merged ones (the GPU-driven one), and rename them to name without Merged. @copilot

[Copilot]

Can completely remove the old per-item pipeline, and only keep the Merged ones (the GPU-driven one), and rename them to name without Merged. @copilot

Done in c247f0a. Removed all old per-item pipeline nodes (VItemComputeNode, VItemDepthNode, VItemColorNode — old per-item versions) and their associated pipeline structs, bind groups, and shaders. The GPU-driven merged nodes are now renamed to VItemComputeNode, VItemDepthNode, VItemColorNode (and pipelines similarly). Also cleaned up VItemRenderInstance/PlaneUniform from primitives/vitem.rs and graph/mod.rs since they were only used by the old path.