Add a render graph framework for the WebGPU renderer

[MatzeOGH]

Adds a render graph to webgpu/base (webgpu::rg). Each frame you declare the
resources and passes you need. The graph compiles them into an execution order, manages
resource lifetimes, and records the passes into your command encoder.

It handles pass ordering and resource lifetime. It is not a GPU-API wrapper! pass bodies
call wgpu* functions directly.

This PR is the library and its tests. Wiring it into webgpu_app (Window, renderers,
overlays, debug panel) is a separate PR #251 stacked on this one.

How it works

You declare resources and get back a TextureHandle or BufferHandle, cheap value types
that name a resource for the current frame. Handles are kind-tagged, so passing a buffer to
sampled() is a compile error rather than a runtime assert.

Passes are declared with add_pass(setup, execute). Setup runs immediately and declares
what the pass reads and writes through a PassBuilder: attachments, sampled and storage
textures and buffers, copies. Execute runs later inside execute(), once passes are
scheduled and resources are allocated. It gets a PassContext that resolves handles to
live WGPUTexture and WGPUBuffer objects and records the actual draw, dispatch, and copy
commands.

Ordering comes from the declared reads and writes. A pass whose output nothing consumes is
culled unless marked force_keep().

Resources can be transient (pooled and aliased onto shared memory when lifetimes don't
overlap), persistent, imported (the swapchain, for example), or history resources that
ping-pong across frames. Render, compute, and transfer passes are supported, along with
subresource and buffer-range targeting and hash-gated rebakes via initialize().

Files

• webgpu/base/RenderGraph.{h,cpp}, RenderGraph_internal.h
• unittests/webgpu_engine/test_RenderGraph.cpp
• docs/rendergraph.md
• CMake registration in webgpu/base and unittests/webgpu_engine

docs/rendergraph.md covers the architecture and has worked examples.

[adam-ce]

Failing pipeline are unrelated unit tests. Review will be for Gerald :)