fix(windows): eliminate the white patch behind MSI launch checkbox

.github/workflows/acceptance.yml

@@ -36,6 +36,9 @@ jobs:
         run: |
           raco pkg install --auto --no-docs "https://github.com/turinglambdaai/keel.git#$env:KEEL_PIN"
           raco pkg install --auto --no-docs "https://github.com/turinglambdaai/rivet.git#$env:RIVET_PIN"
+      - name: Verify Windows installer artwork
+        shell: pwsh
+        run: python scripts/check-windows-installer-bitmaps.py
       - name: Run executable acceptance contract
         shell: pwsh
         run: |

.github/workflows/ci.yml

@@ -25,6 +25,8 @@ jobs:
         run: bash scripts/check-release-version.sh
       - name: Verify dependency pins stay aligned
         run: bash scripts/check-dependency-pins.sh
+      - name: Verify Windows installer bitmap and checkbox surface
+        run: python3 scripts/check-windows-installer-bitmaps.py
 
   core:
     name: Racket core (contract tests)

.github/workflows/release.yml

@@ -102,6 +102,9 @@ jobs:
       - name: Version preflight (VERSION == rivet.rktd == updater == tag)
         shell: bash
         run: bash scripts/check-release-version.sh "${{ github.ref_name }}"
+      - name: Verify Windows installer checkbox surface
+        shell: pwsh
+        run: python scripts/check-windows-installer-bitmaps.py
       - name: Release (MSI + portable zip)
         shell: pwsh
         run: raco rivet release --without-updates --development

scripts/check-windows-installer-bitmaps.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+"""Verify WiX 5 completion dialog bitmap and native checkbox background."""
+from __future__ import annotations
+
+import struct
+import zlib
+from pathlib import Path
+
+ASSETS = Path(__file__).resolve().parents[1] / "assets"
+WIDTH, HEIGHT = 493, 312
+
+
+def bmp_pixels() -> bytes:
+    source = (ASSETS / "wizard-dialog.bmp").read_bytes()
+    assert source[:2] == b"BM", "invalid BMP"
+    offset = struct.unpack_from("<I", source, 10)[0]
+    width, height, planes, depth = struct.unpack_from("<iiHH", source, 18)
+    assert (width, abs(height), planes, depth) == (WIDTH, HEIGHT, 1, 24)
+    stride = (WIDTH * 3 + 3) & ~3
+    assert offset + HEIGHT * stride <= len(source)
+    pixels = bytearray()
+    for y in range(HEIGHT):
+        row = y if height < 0 else HEIGHT - y - 1
+        offset_row = offset + row * stride
+        for x in range(WIDTH):
+            start = offset_row + x * 3
+            blue, green, red = source[start : start + 3]
+            pixels.extend((red, green, blue))
+    return bytes(pixels)
+
+
+def png_pixels() -> bytes:
+    source = (ASSETS / "wizard-dialog.png").read_bytes()
+    assert source[:8] == b"\x89PNG\r\n\x1a\n"
+    pos, chunks = 8, []
+    while pos < len(source):
+        size = struct.unpack_from(">I", source, pos)[0]
+        tag = source[pos + 4 : pos + 8]
+        payload = source[pos + 8 : pos + 8 + size]
+        if tag == b"IHDR":
+            assert struct.unpack_from(">IIBBBBB", payload) == (
+                WIDTH, HEIGHT, 8, 2, 0, 0, 0
+            )
+        if tag == b"IDAT":
+            chunks.append(payload)
+        pos += size + 12
+    raw = zlib.decompress(b"".join(chunks))
+    stride = WIDTH * 3
+    pixels = bytearray()
+    for y in range(HEIGHT):
+        offset = y * (stride + 1)
+        assert raw[offset] == 0, "PNG preview must use filter None"
+        pixels.extend(raw[offset + 1 : offset + 1 + stride])
+    return bytes(pixels)
+
+
+def main() -> None:
+    pixels = bmp_pixels()
+    assert png_pixels() == pixels, "preview PNG differs from shipping BMP"
+    row_width = WIDTH * 3
+    for y in range(248, HEIGHT):
+        row = pixels[y * row_width : (y + 1) * row_width]
+        assert row == bytes((240, 240, 240)) * WIDTH, (
+            f"Windows checkbox backing is not flat #F0F0F0 at y={y}"
+        )
+    divider = pixels[247 * row_width : 248 * row_width]
+    assert divider == bytes((214, 210, 206)) * WIDTH
+    print("Windows MSI artwork OK: 493x312, flat native checkbox footer")
+
+
+if __name__ == "__main__":
+    main()

shared/spec/DESIGN-TOKENS.md

@@ -109,3 +109,18 @@ accent-filled white-text rounded button. No chrome field borders anywhere.
 White rounded square (macOS-squircle radius ≈ 22.5%) with a quiet checklist
 mark: one system-blue rounded bar over two neutral (`#CECED3`) bars, staggered
 widths. Monochrome, no gradients — must read on light and dark docks alike.
+
+## Windows MSI installer artwork
+
+WiX 5's `WixUI_InstallDir` shares `assets/wizard-dialog.bmp` (493×312)
+between welcome and completion pages. Its `ExitDialog` optional checkbox is
+**opaque** at x=135…355 and y=190…230 installer units, mapping to approximately
+x=180…473 and y=253…307 on the bitmap. Gradients under this native control
+produce a conspicuous rectangular patch.
+
+The image's **entire bottom footer** (y=248…311) is flat classic Windows
+button-face `#F0F0F0`, with a subtle divider at y=247. The interactive
+checkbox remains native and accessible, while its backing now blends into the
+same surface. Keep SVG, PNG preview, and shipping BMP synchronized.
+`scripts/check-windows-installer-bitmaps.py` verifies pixel identity and
+native checkbox surface in CI, packaged acceptance and Windows release.