chore: import upstream snapshot with attribution

packaging/macos/README-macOS.txt

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+StemDeck for macOS
+==================
+
+Install:
+
+1. Open the StemDeck DMG.
+2. Drag StemDeck.app to Applications.
+3. Open StemDeck from Applications.
+
+First launch:
+
+- StemDeck is a thin native app. It downloads a pinned, checksummed StemDeck
+  runtime pack on first launch.
+- The runtime installs to:
+  ~/Library/Application Support/StemDeck/runtime
+- FFmpeg and ffprobe install to:
+  ~/Library/Application Support/StemDeck/ffmpeg
+- Demucs model weights download on first use and are cached under:
+  ~/Library/Application Support/StemDeck/models
+
+Uninstall:
+
+1. Delete /Applications/StemDeck.app.
+2. To remove runtime files, jobs, caches, models, and logs, delete:
+   ~/Library/Application Support/StemDeck
+
+Notes:
+
+- Internet access is required for first-run setup.
+- Public releases should be signed and notarized.
+- Unsigned local builds are for development and internal testing only.