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

.gitea/workflows/deploy.yaml

@@ -79,7 +79,27 @@ jobs:
           sudo apt-get update -qq
           sudo apt-get install -y --no-install-recommends build-essential libcurl4-openssl-dev libssl-dev
 
-      - name: Build elc fresh on the runner
+      - name: Restore elc binary from GCS cache
+        id: elc-cache
+        run: |
+          set -euo pipefail
+          # Cache key = SHA256 of both source files that produce elc
+          ELC_SRC_HASH=$(cat "$EL_HOME/dist/platform/elc.c" "$EL_HOME/el-compiler/runtime/el_runtime.c" | sha256sum | cut -c1-16)
+          CACHE_OBJ="gs://neuron-ci-cache/elc/linux-amd64/${ELC_SRC_HASH}/elc"
+          echo "elc_cache_key=${ELC_SRC_HASH}" >> "$GITHUB_OUTPUT"
+          echo "elc_cache_obj=${CACHE_OBJ}" >> "$GITHUB_OUTPUT"
+          if gsutil -q stat "$CACHE_OBJ" 2>/dev/null; then
+            echo "Cache hit: ${CACHE_OBJ}"
+            gsutil cp "$CACHE_OBJ" "$EL_HOME/dist/platform/elc"
+            chmod +x "$EL_HOME/dist/platform/elc"
+            echo "cache_hit=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "Cache miss for key ${ELC_SRC_HASH}"
+            echo "cache_hit=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Build elc (cache miss only)
+        if: steps.elc-cache.outputs.cache_hit != 'true'
         run: |
           set -euo pipefail
           # The committed dist/platform/elc is arm64 from Will's mac. We
@@ -93,6 +113,8 @@ jobs:
             -lcurl -lpthread -ldl -lm -lssl -lcrypto
           file dist/platform/elc
           ls -la dist/platform/elc
+          # Upload to GCS cache for future runs
+          gsutil cp dist/platform/elc "${{ steps.elc-cache.outputs.elc_cache_obj }}" || true
 
       - name: Compute image tag
         id: tag