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>

Cargo.lock

@@ -21,6 +21,7 @@ dependencies = [
  "clap",
  "console_error_panic_hook",
  "cosmic-text",
+ "dwg-thumbnailer",
  "env_logger",
  "flate2",
  "fontdb",
@@ -1414,6 +1415,21 @@ version = "0.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "edf234dd1594d6dd434a8fb8cada51ddbbc593e40e4a01556a0b31c62da2775b"
 
+[[package]]
+name = "dwg-thumbnailer"
+version = "0.1.0"
+dependencies = [
+ "image",
+]
+
+[[package]]
+name = "dwg-thumbnailer-win"
+version = "0.1.0"
+dependencies = [
+ "dwg-thumbnailer",
+ "windows 0.58.0",
+]
+
 [[package]]
 name = "ecb"
 version = "0.1.2"