Add packed integer dot product sample

[lyonsno]

Adds a small dot4I8Packed compute example toward #350.

The sample packs four signed 8-bit components into each u32, computes four example dot products, and reads the results back from the GPU. A dat.gui selector displays the input vectors and result for each example.

It requires packed_4x8_integer_dot_product and displays a requirement message when that WGSL language feature is unavailable. There is one packed compute pipeline, with no scalar fallback or in-sample result validation.

Testing

• Lint, source typecheck, and the Node 20.20.2 build pass. The build emits existing typed-array warnings in unrelated samples.
• Chrome 153.0.8010.48, headless with --enable-unsafe-webgpu, on an Apple Metal adapter: all four examples returned the expected results, and the dat.gui selector was exercised.
• Desktop and narrow-screen layouts inspected. Injecting feature unavailability displayed the requirement message without requesting an adapter or compiling a shader.

No new dependencies or external assets.

[kainino0x]

Decided to add back a GUI to let people actually play with the inputs of the function, and remove the batched execution because there's just one input. It's a really barebones sample so will be interested still in replacing it at some point.