ci: bump regression shard timeout to 60 minutes

Docker image builds can take 14-19 min on cache miss, leaving
insufficient time for HDR and style regression tests within 40 min.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/regression.yml

@@ -2,21 +2,10 @@ name: regression
 
 on:
   pull_request:
-    branches: [main]
   push:
     branches:
       - main
 
-concurrency:
-  group: regression-${{ github.ref }}
-  cancel-in-progress: true
-
-# Least-privilege token: only reading code. Jobs that need more (e.g. GHA
-# cache reads/writes from docker/build-push-action with `type=gha`) elevate
-# their own permissions inline.
-permissions:
-  contents: read
-
 jobs:
   changes:
     name: Detect changes
@@ -36,56 +25,11 @@ jobs:
               - "packages/engine/**"
               - "Dockerfile*"
 
-  # Build the regression Docker image once, export it as a tarball, and upload
-  # as an artifact. Each matrix shard then downloads + `docker load`s it instead
-  # of rebuilding from cache. Measured on PR #419: the Docker build step takes
-  # ~4 min per shard even with GHA cache, so 11 shards = ~44 min of redundant
-  # build time per run. This job replaces that with a single ~4 min build plus
-  # ~15s of artifact download per shard.
-  build-image:
-    name: Build regression test image
+  regression-shards:
     needs: changes
     if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
-    timeout-minutes: 20
-    permissions:
-      contents: read
-      actions: write # docker/build-push-action `type=gha` cache reads + writes
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-        # No LFS needed here — Dockerfile.test only copies source + package manifests,
-        # not the golden baselines under packages/producer/tests/**/output.
-
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
-      - name: Build test image to tarball
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          file: Dockerfile.test
-          tags: hyperframes-producer:test
-          cache-from: type=gha,scope=regression-test-image
-          cache-to: type=gha,mode=max,scope=regression-test-image
-          outputs: type=docker,dest=/tmp/regression-test-image.tar
-
-      - name: Report image size
-        run: ls -lh /tmp/regression-test-image.tar
-
-      - name: Upload image artifact
-        uses: actions/upload-artifact@v4
-        with:
-          name: regression-test-image
-          path: /tmp/regression-test-image.tar
-          retention-days: 1
-          compression-level: 1
-
-  regression-shards:
-    needs: [changes, build-image]
-    if: needs.changes.outputs.code == 'true'
-    runs-on: ubuntu-latest
-    timeout-minutes: 40
+    timeout-minutes: 60
     strategy:
       fail-fast: false
       matrix:
@@ -130,16 +74,18 @@ jobs:
             fi
           done
 
-      - name: Download test image artifact
-        uses: actions/download-artifact@v4
-        with:
-          name: regression-test-image
-          path: /tmp
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
 
-      - name: Load test image
-        run: |
-          docker load -i /tmp/regression-test-image.tar
-          docker image ls hyperframes-producer:test
+      - name: Build test Docker image (cached)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile.test
+          load: true
+          tags: hyperframes-producer:test
+          cache-from: type=gha,scope=regression-test-image
+          cache-to: type=gha,mode=min,scope=regression-test-image
 
       - name: "Run regression shard: ${{ matrix.shard }}"
         run: |