Make downsample.wesl valid WESL

[stuartparmenter]

Objective

Fixes #25733. We were doing a find-and-replace on ##TEXTURE_FORMAT## in downsample.wesl at runtime, which meant the file was never valid WESL.

Solution

• Specialize the shader with shader defs instead. It declares one @if(TEXTURE_FORMAT_*) block per supported format, and ARRAY_TEXTURE picks the 2D or array variant.
• DownsampleShaders::general is now a single Handle<Shader>, specialized lazily by the pipeline cache instead of 40 shader assets created at startup.
• The shader is specialized through a new DownsamplePipeline resource and SpecializedComputePipelines<DownsamplePipeline>, like the other compute pipelines. The light probe generator uses it instead of its own copy of the layouts and defs.
• Renamed the FIRST_PASS and SECOND_PASS shader defs to SPLIT_BIND_GROUP_FIRST and SPLIT_BIND_GROUP_SECOND, since exactly one of those and COMBINE_BIND_GROUP is set.

I don't love the boilerplate in this approach, but don't see a better solution. I did prototype a working constants:: version too, but rejected it since it seems like we're generally trying to move away from them.

Testing

dynamic_mip_generation and reflection_probes render correctly.

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Used Claude Code & Fable 5.1 to help explore various ways to do this, build tests to see what naga supports and what wesl supports (and what they disagree on) to try various approaches, then to build out multiple approaches, which I then reviewed, settled on this one, validated questions I had, cleaned up PR a bit, then wrote migration guide and here we are.

[CodingDaniel1]

I have also found discrepencies related to pipeline creation (light_probe) against other areas in bevy (clustering, binning). But thats outside the scope of this PR. Which could be follow up work.

Overall this pr is good imo. Except the weird naming pre-existed.

[CodingDaniel1]

Looks good to me