fix(ci): kill the remaining transient-failure classes in workflows + Dockerfile

From the workflow reliability audit:
- tests.yml: duration-cache restore had NO restore-keys while saves use
  run_id-suffixed keys — the cache never matched once, so LPT slicing
  always ran blind and unbalanced slices pushed heavy files toward the
  per-file timeout. One-line restore-keys fixes slice balancing.
- Label gates (lint ci-reviewed, supply-chain mcp-catalog-reviewed):
  'gh pr view || true' turned an API blip into 'label absent' → false
  BLOCKING failure. Now 3x retry, and API failure is reported as an API
  failure instead of a missing label.
- detect-changes action: compare API retried before failing open (was
  silently running all lanes on any blip).
- uv-lockfile-check: 'uv lock --check' resolves against PyPI — retried
  so registry blips don't read as 'lockfile stale'.
- docker.yml merge job: imagetools create retried (Docker Hub eventual
  consistency on just-pushed digests).
- Dockerfile: apt-get Acquire::Retries=3; s6-overlay ADDs converted to
  curl --retry 3 (ADD cannot retry; checksums still enforced); npm
  --fetch-retries=5; playwright chromium fetch retried 3x.
- Advisory artifact uploads (per-slice durations, ci-timings report)
  get continue-on-error so an artifact-service blip can't fail a green
  test slice.

.github/workflows/docker.yml

@@ -189,15 +189,23 @@ jobs:
             args+=("${IMAGE_NAME}@sha256:${digest_file}")
           done
           if [ "${{ github.event_name }}" = "release" ]; then
-            docker buildx imagetools create \
-              -t "${IMAGE_NAME}:${RELEASE_TAG}" \
-              "${args[@]}"
+            tags=(-t "${IMAGE_NAME}:${RELEASE_TAG}")
           else
-            docker buildx imagetools create \
-              -t "${IMAGE_NAME}:main" \
-              -t "${IMAGE_NAME}:latest" \
-              "${args[@]}"
+            tags=(-t "${IMAGE_NAME}:main" -t "${IMAGE_NAME}:latest")
           fi
+          # Retry: Docker Hub API + just-pushed digest eventual consistency
+          # can transiently fail the create; the operation is idempotent.
+          for i in 1 2 3; do
+            if docker buildx imagetools create "${tags[@]}" "${args[@]}"; then
+              break
+            fi
+            if [ "$i" = 3 ]; then
+              echo "::error::imagetools create failed after 3 attempts"
+              exit 1
+            fi
+            echo "::warning::imagetools create failed (attempt $i); retrying in 20s"
+            sleep 20
+          done
 
       - name: Inspect image
         env: