ci: add asset-only fast path (<5min for PNG changes)

- Detect asset-only changes (src/assets/, src/shares/, static HTML, llms.txt)
  and skip El compilation, C build deps, and Docker full build entirely
- Fast path pulls :latest as base and rebuilds only the assets layer
- Gate clone-el, install-C-deps, elc-cache, build-elc, build-image, push-image
  behind asset_only != 'true'; deploy steps run unconditionally
- Switch build-stage.sh from registry cache driver (requires docker-container
  buildx driver) to inline cache backed by :latest — compatible with default
  docker driver on the runner

build-stage.sh

@@ -89,8 +89,8 @@ sed "${SED_INPLACE[@]}" \
 
 echo "==> Building Docker image marketing:${TAG} for linux/amd64"
 docker buildx build --platform linux/amd64 --load \
-    --cache-from type=registry,ref=us-central1-docker.pkg.dev/neuron-785695/neuron-marketing/marketing:cache \
-    --cache-to type=registry,ref=us-central1-docker.pkg.dev/neuron-785695/neuron-marketing/marketing:cache,mode=max \
+    --build-arg BUILDKIT_INLINE_CACHE=1 \
+    --cache-from us-central1-docker.pkg.dev/neuron-785695/neuron-marketing/marketing:latest \
     -f Dockerfile.stage \
     -t "marketing:${TAG}" \
     .