Feature/skainet 0.40 0.39 migration kernelrace - #30

AndroidNeonLlmDemo showed SKaiNET's hand-written ARM NEON kernels with an
Android-only NEON-vs-scalar A/B race, but every other sample in this repo is
Kotlin Multiplatform. This turns it into KernelRace: Android stays the focus
(the race itself is inherently Android-only kernel-SPI territory), and the
same engine now also runs on Desktop and in the browser as a KMP app. iOS is
deferred until the multiplatform structure is proven out.

- shared/: engine, model resolution and ChatViewModel are platform-neutral
  commonMain code with expect/actual seams per platform (LlamaRuntimeBuilder
  for runtime construction, ModelProvider for Android download/cache/asset
  vs. Desktop cache vs. Wasm's bundled-at-build-time bytes). ChatViewModel
  depends on a GenerativeEngine interface and an injectable dispatcher
  specifically so it's unit-testable with a fake on every target.
- composeApp/: shared UI on the skainet-ui design system, with a
  kernelControls slot so the Android-only race UI (KernelRaceApp's kernel
  pinning, the NEON/SCALAR chips, the split-screen race button) never leaks
  into common code.
- Real test coverage: PromptTemplate/TokenStats/ModelResolver unit tests, a
  ChatViewModel test suite (fake engine, virtual-time coroutine scheduler),
  and a GgufSmokeTest that loads the actual SmolLM2 GGUF and generates
  tokens end to end when the model is present locally.
- CI workflow (first build CI in this repo): JVM/Android unit tests, Android
  debug APK assembly, and a wasm compile check on every PR touching
  KernelRace/ or skainet-ui/.

Verified end to end: clean builds pass on Android/JVM/wasmJs, the Android
debug APK packages the NEON .so libs, and the wasm production webpack build
correctly bundles the ~145 MB GGUF via Compose resources.

The original AndroidNeonLlmDemo snapshot remains available at the
2026_08_arm_android git tag.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

The samples page only ever showed a card for samples with a wasm build:
generate-samples-page.py unconditionally emitted a demoUrl for every
webapp.json it found. CLI-only and Android-only samples had no way to
appear at all.

- distDirs/build are now optional; a card only needs id/name/description.
- demoUrl is emitted only when distDirs is non-empty; samples without one
  get a "How to run" button linking to the source instead of a dead "Try
  Demo".
- New "platforms" array renders as chips on every card (web, android, ios,
  desktop, cli, ...), reusing the existing pill styling.
- Added platforms to the four existing webapp.json files and a new
  MnistJavaDemo/webapp.json (the first CLI-only sample on the page).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

The repo's root .gitignore excludes *.jar, which silently dropped
gradle-wrapper.jar when KernelRace was created (AndroidNeonLlmDemo never had
it committed either -- it just never ran in CI). Every sample that runs in
CI (TinyTransformer, KllamaDemo, ...) force-adds this jar past the .gitignore
rule; KernelRace needs the same, or ./gradlew has nothing to execute.

Fixes the KernelRace CI failure on PR #30: "Unable to access jarfile
.../KernelRace/gradle/wrapper/gradle-wrapper.jar".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>