docs: close out Milestone 9 and document Windows/macOS packaging

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

docs/BUILDING.md

@@ -106,3 +106,37 @@ sudo apt-get install -y flatpak flatpak-builder
 
 Output path:
 - `dist/flatpak/org.darksingularity.OrbitHub.flatpak`
+
+## Windows Packaging
+
+Requires [Inno Setup 6](https://jrsoftware.org/isinfo.php) (`winget install -e --id JRSoftware.InnoSetup`).
+
+From a configured and built `build\` directory (see Windows build steps above):
+
+```powershell
+.\packaging\windows\build-installer.ps1
+```
+
+Output path:
+- `dist\windows\OrbitHub-Setup-<version>.exe`
+
+## macOS Packaging
+
+Requires `qt@6` from Homebrew (for `macdeployqt`). For a custom volume icon on
+the `.dmg`, also install `SetFile` (via Xcode's "Additional Tools", from
+[developer.apple.com/download/all](https://developer.apple.com/download/all))
+or `brew install fileicon` — packaging still works without either, just with
+the generic disk image icon.
+
+```bash
+./packaging/macos/build-dmg.sh
+```
+
+This runs `cmake --install`, then `macdeployqt` to bundle Qt frameworks and
+plugins into the `.app`, then re-signs the bundle (ad hoc, since there is no
+Developer ID certificate) so it passes macOS's launch-time code-integrity
+check. The result is unsigned/unnotarized, so first launch requires
+right-click → **Open** to bypass Gatekeeper's unidentified-developer warning.
+
+Output path:
+- `dist/macos/OrbitHub-<version>.dmg`