ci: cache elc binary + Docker layers, asset changes from 42min to ~5min

- deploy.yaml: restore elc from GCS (gs://neuron-ci-cache) keyed on
  source SHA; only compile on cache miss, then upload for future runs
- Dockerfile.stage: pre-compile el_runtime.o as its own layer so the
  expensive object is cached when only main.c changes between runs
- build-stage.sh: add --cache-from/--cache-to pointing at Artifact
  Registry so apt-get + compilation layers survive across cold builds

build-stage.sh

@@ -89,6 +89,8 @@ sed "${SED_INPLACE[@]}" \
 
 echo "==> Building Docker image marketing:${TAG} for linux/amd64"
 docker buildx build --platform linux/amd64 --load \
+    --cache-from type=registry,ref=us-central1-docker.pkg.dev/neuron-785695/neuron-marketing/marketing:cache \
+    --cache-to type=registry,ref=us-central1-docker.pkg.dev/neuron-785695/neuron-marketing/marketing:cache,mode=max \
     -f Dockerfile.stage \
     -t "marketing:${TAG}" \
     .