feat(snap): Snap Store packaging for Open CAD Studio

Repackages the released Linux AppImage (--appimage-extract) instead of
rebuilding from source, triggered on completion of the "Release" workflow.
Publishing to the Snap Store only happens once SNAPCRAFT_STORE_CREDENTIALS
is set as a repo secret.

Different from the other OpenAEC snaps in one important way: OCS uses
winit + wgpu (Vulkan/GL), not GTK/WebKit, so this uses explicit
opengl/wayland/x11 plugs and staged Mesa/X11/Wayland runtime libraries
instead of the `gnome` extension.

Higher confidence in the packaging mechanics (build succeeds, produces a
valid .snap) than in the actual runtime GPU/windowing behavior under
strict confinement, which needs real hands-on testing on a Linux desktop
before publishing to stable -- see the PR description.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

snap/snapcraft.yaml

@@ -0,0 +1,84 @@
+name: open-cad-studio
+base: core22
+# Versie wordt in CI overschreven met het release-versienummer (zie .github/workflows/snap.yml).
+version: '0.0.0'
+title: Open CAD Studio
+summary: 2D drafting and 3D modeling with native DWG/DXF support
+description: |
+  Open CAD Studio (OCS) is a CAD application for 2D drafting and 3D
+  modeling, built in Rust. Reads and writes DWG and DXF files natively
+  (R13 through R2018).
+
+  File formats:
+  - DWG and DXF read/write (R13-R2018)
+  - STL export, STEP AP203 export, OBJ import
+  - PDF export (plot layouts)
+  - WBLOCK, and XREF (attach/reload/auto-resolve external references)
+
+  2D drafting:
+  - Lines, polylines, rectangles, polygons, circles, arcs, ellipses,
+    splines
+  - Hatch fills with pattern, scale, and angle editing
+  - Offset, trim, extend, fillet, break, stretch, lengthen
+  - Array, mirror, move, copy, rotate, scale, explode
+  - Double-click text editing, and area/perimeter/centroid measurement
+
+  3D modeling:
+  - Solid primitives (box, sphere, cylinder), extrude, revolve, loft,
+    sweep, and 3D array
+  - ACIS tessellation for rendering 3DSOLID/REGION/BODY entities
+
+  Open CAD Studio is open source, part of the OpenAEC Foundation family
+  of applications, and rendered with wgpu for GPU-accelerated 2D/3D
+  drawing.
+
+grade: stable
+confinement: strict
+
+apps:
+  open-cad-studio:
+    command: usr/bin/OpenCADStudio
+    desktop: usr/share/applications/io.github.HakanSeven12.OpenCadStudio.desktop
+    plugs:
+      - home            # tekeningen openen/opslaan/exporteren in de home-map
+      - removable-media # idem op externe schijven/USB
+      - network         # licentie-/Patreon-check
+      - opengl          # GPU-toegang voor wgpu (Vulkan/GL)
+      - wayland
+      - x11
+      - desktop
+      - desktop-legacy
+
+parts:
+  open-cad-studio:
+    plugin: nil
+    # De CI-stap kopieert de gepubliceerde release-AppImage naar de project-root
+    # als `open-cad-studio.AppImage`; we pakken die uit i.p.v. opnieuw te bouwen
+    # (dezelfde herverpak-aanpak als de .deb-gebaseerde OpenAEC-snaps, maar dan
+    # voor een AppImage: `--appimage-extract` levert een kant-en-klare AppDir).
+    override-build: |
+      set -eu
+      chmod +x "${CRAFT_PROJECT_DIR}/open-cad-studio.AppImage"
+      "${CRAFT_PROJECT_DIR}/open-cad-studio.AppImage" --appimage-extract
+      mkdir -p "${CRAFT_PART_INSTALL}/usr"
+      cp -r squashfs-root/usr/. "${CRAFT_PART_INSTALL}/usr/"
+    # OCS gebruikt winit (X11/Wayland) + wgpu (Vulkan met GL-fallback), geen
+    # GTK/WebKit — dus GEEN `extensions: [gnome]` (dat is voor de Tauri-apps).
+    # libvulkan1/mesa-vulkan-drivers zijn een fallback; echte hardware-versnelling
+    # loopt via de `opengl`-plug naar de host-driver.
+    stage-packages:
+      - libvulkan1
+      - mesa-vulkan-drivers
+      - libgl1
+      - libx11-6
+      - libxcursor1
+      - libxi6
+      - libxrandr2
+      - libxinerama1
+      - libxkbcommon0
+      - libxkbcommon-x11-0
+      - libwayland-client0
+      - libwayland-cursor0
+      - libwayland-egl1
+      - libfontconfig1
+      - libfreetype6