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>

packaging/windows/main.wxs

@@ -84,6 +84,20 @@
                 Advertise='yes' />
             </File>
 
+            <!-- DWG/DXF file-type icons, generated from the mimetype SVGs
+                 (CI: magick image-vnd.{dwg,dxf}.svg -> {dwg,dxf}.ico). -->
+            <File Id='DwgIco' Name='dwg.ico' DiskId='1' Source='packaging\windows\dwg.ico' />
+            <File Id='DxfIco' Name='dxf.ico' DiskId='1' Source='packaging\windows\dxf.ico' />
+
+            <!-- DWG thumbnail provider (IThumbnailProvider). Ships beside the
+                 exe; OpenCADStudio self-registers it under HKCU on startup
+                 (see file_association::install_thumbnailer). -->
+            <File
+              Id='DwgThumbDll'
+              Name='dwg_thumbnailer_win.dll'
+              DiskId='1'
+              Source='target\release\dwg_thumbnailer_win.dll' />
+
             <!--
               File associations. Two ProgIDs (one per extension) so the
               `<Verb Id='open'>` rows don't collide on the same registry
@@ -93,7 +107,7 @@
             <ProgId
               Id='OpenCADStudio.DWG'
               Description='DWG Drawing'
-              Icon='OpenCADStudioEXE'
+              Icon='DwgIco'
               IconIndex='0'>
               <Extension Id='dwg' ContentType='image/vnd.dwg'>
                 <Verb Id='open' Command='Open' TargetFile='OpenCADStudioEXE' Argument='"%1"' />
@@ -103,7 +117,7 @@
             <ProgId
               Id='OpenCADStudio.DXF'
               Description='DXF Drawing'
-              Icon='OpenCADStudioEXE'
+              Icon='DxfIco'
               IconIndex='0'>
               <Extension Id='dxf' ContentType='image/vnd.dxf'>
                 <Verb Id='open' Command='Open' TargetFile='OpenCADStudioEXE' Argument='"%1"' />