feat(thumbnails): embed DWG previews and wire cross-platform file thumbnails

Round-trip a raster preview through the DWG's embedded preview slot and
surface it to the OS file managers, so drawings show their contents in
Explorer / Finder / Nautilus instead of a generic document icon.

OCS:
* io::thumbnail — rasterize the scene to a BMP DIB on save (embedded via
  acadrust's new Preview type) and read it back on open for the Start
  page recent list; `--dwg-thumbnail IN OUT SIZE` CLI extracts a badged
  PNG for external thumbnailers.
* io::file_association::install_thumbnailer — self-install the OS
  integration on startup: Linux writes a .thumbnailer + hicolor mimetype
  icons; Windows registers the IThumbnailProvider DLL under HKCU.

Shared core:
* crates/dwg-thumbnailer — lean (image-only) preview extractor + the
  `badge_dwg` full-width "DWG" band, used by every platform so the
  ribbon is single-sourced. Ships an rlib+staticlib.
* crates/dwg-thumbnailer-win — IThumbnailProvider COM in-proc server
  (cfg(windows), CI-built as a workspace member).
* macos/ — QuickLook thumbnail extension (Swift + C-ABI bridge to the
  core staticlib), assembled into a .appex.

Icons & packaging (single SVG source -> per-platform assets in CI):
* assets/mimetypes/image-vnd.{dwg,dxf}.svg — themed file icons.
* packaging: WiX ships dwg/dxf.ico + the thumbnail DLL and points the
  ProgIds at them; Info.plist gets CFBundleTypeIconFile + the QuickLook
  extension in Contents/PlugIns.
* release.yml — generate .ico (Windows) and .icns (macOS) from the SVGs,
  build the QuickLook .appex, and bundle everything.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/Info.plist

@@ -35,6 +35,8 @@
             <string>AutoCAD Drawing</string>
             <key>CFBundleTypeRole</key>
             <string>Editor</string>
+            <key>CFBundleTypeIconFile</key>
+            <string>DWG</string>
             <key>LSItemContentTypes</key>
             <array>
                 <string>com.autodesk.dwg</string>
@@ -47,6 +49,8 @@
             <string>AutoCAD DXF</string>
             <key>CFBundleTypeRole</key>
             <string>Editor</string>
+            <key>CFBundleTypeIconFile</key>
+            <string>DXF</string>
             <key>LSItemContentTypes</key>
             <array>
                 <string>com.autodesk.dxf</string>