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>

crates/dwg-thumbnailer/macos/Info.plist

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<!-- Info.plist for the DWG QuickLook Thumbnail Extension (.appex). -->
+<plist version="1.0">
+<dict>
+    <key>CFBundleDevelopmentRegion</key>
+    <string>en</string>
+    <key>CFBundleDisplayName</key>
+    <string>DWG Thumbnails</string>
+    <key>CFBundleExecutable</key>
+    <string>DWGThumbnail</string>
+    <!-- Must be prefixed with the host app id so it registers as its extension. -->
+    <key>CFBundleIdentifier</key>
+    <string>io.github.HakanSeven12.OpenCadStudio.DWGThumbnail</string>
+    <key>CFBundleInfoDictionaryVersion</key>
+    <string>6.0</string>
+    <key>CFBundleName</key>
+    <string>DWGThumbnail</string>
+    <key>CFBundlePackageType</key>
+    <string>XPC!</string>
+    <key>CFBundleShortVersionString</key>
+    <string>__VERSION__</string>
+    <key>CFBundleVersion</key>
+    <string>__VERSION__</string>
+    <key>NSExtension</key>
+    <dict>
+        <key>NSExtensionPointIdentifier</key>
+        <string>com.apple.quicklook.thumbnail</string>
+        <key>NSExtensionPrincipalClass</key>
+        <string>DWGThumbnail.ThumbnailProvider</string>
+        <key>NSExtensionAttributes</key>
+        <dict>
+            <key>QLSupportedContentTypes</key>
+            <array>
+                <string>com.autodesk.dwg</string>
+            </array>
+            <key>QLThumbnailMinimumDimension</key>
+            <integer>0</integer>
+        </dict>
+    </dict>
+</dict>
+</plist>

crates/dwg-thumbnailer/macos/ThumbnailProvider.swift

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+// macOS QuickLook thumbnail extension for DWG files.
+//
+// Calls the Rust core (`dwg_thumbnail_png`, linked from libdwg_thumbnailer.a)
+// to extract the embedded preview as PNG, then hands it to QuickLook.
+//
+// Add this file to a "Thumbnail Extension" target, expose the C ABI through a
+// bridging header that #includes `dwg_thumbnailer.h`, and link the static lib
+// (see README). QLSupportedContentTypes must list the DWG UTI (com.autodesk.dwg).
+
+import QuickLookThumbnailing
+import CoreGraphics
+import ImageIO
+import Foundation
+
+final class ThumbnailProvider: QLThumbnailProvider {
+    override func provideThumbnail(
+        for request: QLFileThumbnailRequest,
+        _ handler: @escaping (QLThumbnailReply?, Error?) -> Void
+    ) {
+        let maxDim = UInt32(max(request.maximumSize.width, request.maximumSize.height))
+
+        var ptr: UnsafeMutablePointer<UInt8>? = nil
+        var len: Int = 0
+        let ok = request.fileURL.path.withCString { cpath in
+            dwg_thumbnail_png(cpath, maxDim, &ptr, &len)
+        }
+        guard ok, let p = ptr, len > 0 else {
+            handler(nil, nil) // no preview → QuickLook falls back
+            return
+        }
+        let data = Data(bytes: p, count: len)
+        dwg_thumbnail_free(p, len)
+
+        guard
+            let src = CGImageSourceCreateWithData(data as CFData, nil),
+            let cg = CGImageSourceCreateImageAtIndex(src, 0, nil)
+        else {
+            handler(nil, nil)
+            return
+        }
+
+        let size = CGSize(width: cg.width, height: cg.height)
+        let reply = QLThumbnailReply(contextSize: size) { (ctx: CGContext) -> Bool in
+            ctx.draw(cg, in: CGRect(origin: .zero, size: size))
+            return true
+        }
+        handler(reply, nil)
+    }
+}

crates/dwg-thumbnailer/macos/dwg_thumbnailer.h

@@ -0,0 +1,18 @@
+/* Bridging header for the macOS QuickLook extension.
+ * Exposes the Rust core's C ABI (built as `libdwg_thumbnailer.a`). */
+#ifndef DWG_THUMBNAILER_H
+#define DWG_THUMBNAILER_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* Extract a DWG preview, encoded as PNG. On success writes a malloc'd buffer to
+ * *out_ptr / *out_len (free with dwg_thumbnail_free) and returns true. */
+bool dwg_thumbnail_png(const char *path_utf8, uint32_t max_dim,
+                       uint8_t **out_ptr, size_t *out_len);
+
+/* Free a buffer returned by dwg_thumbnail_png. */
+void dwg_thumbnail_free(uint8_t *ptr, size_t len);
+
+#endif /* DWG_THUMBNAILER_H */