fix(macos): center the drag arrow between DMG installer icons

.github/workflows/acceptance.yml

@@ -74,6 +74,10 @@ jobs:
         run: |
           raco pkg install --auto --no-docs "https://github.com/turinglambdaai/keel.git#${KEEL_PIN}"
           raco pkg install --auto --no-docs "https://github.com/turinglambdaai/rivet.git#${RIVET_PIN}"
+      - name: Render DMG artwork from SVG
+        run: |
+          brew install librsvg
+          bash scripts/render-dmg-background.sh
       - name: Run executable acceptance contract
         shell: bash
         run: |

.github/workflows/release.yml

@@ -45,6 +45,12 @@ jobs:
           raco pkg install --auto --no-docs 'https://github.com/turinglambdaai/rivet.git#${{ env.RIVET_PIN }}'
       - name: Version preflight (VERSION == rivet.rktd == updater == tag)
         run: bash scripts/check-release-version.sh "${{ github.ref_name }}"
+      # The Finder DMG uses the PNG, not the editable SVG. Regenerate it so
+      # artwork changes always land in the release (Rivet uses a 660x400 view).
+      - name: Render DMG artwork from SVG
+        run: |
+          brew install librsvg
+          bash scripts/render-dmg-background.sh
       # DMG (human installer) + portable zip + SBOM + notices, launch
       # smoke included. The macOS update feed entry is the portable zip,
       # assembled by scripts/make-update-manifest.sh in the publish job.

scripts/render-dmg-background.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# The SVG is the editable source; Rivet stages the PNG named in rivet.rktd.
+# Run before packaging on macOS so the release never ships stale artwork.
+set -euo pipefail
+root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+svg="$root/assets/dmg-background.svg"
+png="$root/assets/dmg-background.png"
+if ! command -v rsvg-convert >/dev/null 2>&1; then
+  echo "rsvg-convert is required (macOS: brew install librsvg)" >&2
+  exit 1
+fi
+rsvg-convert --format png --output "$png" "$svg"
+# Fail before Rivet if the generated image no longer matches its fixed Finder
+# window geometry (660x400 at 2x): a valid PNG may still crop/misalign icons.
+python3 - "$png" <<'PY'
+import struct
+import sys
+with open(sys.argv[1], "rb") as stream:
+    header = stream.read(24)
+if header[:8] != b"\x89PNG\r\n\x1a\n" or header[12:16] != b"IHDR":
+    raise SystemExit("DMG artwork is not a valid PNG")
+size = struct.unpack(">II", header[16:24])
+if size != (1320, 800):
+    raise SystemExit(f"DMG artwork must be 1320x800, got {size}")
+print(f"Rendered DMG artwork: {size[0]}x{size[1]}")
+PY