Ship Linux x86_64 + NVIDIA support

[brxs]

Summary

Ship a supported Linux desktop build for Ubuntu 22.04 or newer on x86_64 systems with a supported NVIDIA GPU, distributed as an AppImage with full LSDJ feature parity.

Dependencies

• Refactor LSDJ's cross-platform application foundation #107
• Add a production PyTorch MRT2 backend for Linux and Windows #110
• Add a portable Stable Audio 3 TFLite backend #111
• Add third-party model licensing, attribution, and download acknowledgement #108

Supported configuration

• Ubuntu 22.04+ x86_64.
• Xorg and Wayland sessions.
• NVIDIA GPU/driver/PyTorch-CUDA floor determined by Spike: validate Apolinario's PyTorch MRT2 port for LSDJ #109 and the production backend qualification.
• Minimum VRAM, CPU, RAM, free disk, and audio configuration documented from measured results. CPU/RAM matter because Stable Audio uses the TFLite baseline.
• AppImage is the only release format in this issue.

Scope

Audio

• Validate Refactor LSDJ's cross-platform application foundation #107's shared sample-format/channel/resampling path on ALSA/PipeWire hosts and make Linux-specific fixes only where required.
• Support default-device selection/change, 48 kHz and non-48 kHz devices, stereo output, and the FLX4 four-channel routing used by LSDJ.
• Recover clearly from device removal, PipeWire restart, suspend/resume, and unsupported format/layout.
• Ensure the callback remains allocation-free and model/service work cannot block it.

MIDI and controller behavior

• Pioneer/AlphaTheta DDJ-FLX4 is the required controller for release qualification.
• DDJ-400 remains best-effort regression coverage unless it is separately advertised as supported.
• Normalize Linux MIDI port names and test hotplug, reconnect, transport, mixer controls, jog wheels, LEDs, and required SysEx behavior.
• Document user-group/udev/permission setup where the distribution requires it, with actionable in-app diagnostics.

Desktop integration

• Validate titlebar/window behavior, file and folder dialogs, notifications, opener/trash integration, XDG paths, and packaged resource lookup.
• Test spaces, Unicode, and non-ASCII home paths.
• Validate both Wayland and Xorg behavior on Ubuntu 22.04+.

Runtime and model experience

• Provide a clean first-run flow that installs verified MRT2 and Stable Audio runtimes/models without system Python, Git, a CUDA toolkit, or shell commands.
• Use the PyTorch MRT2 backend and TFLite Stable Audio baseline.
• Show backend, model/runtime state, required storage, driver incompatibility, download verification, and recovery errors clearly.
• Protect live decks from Stable Audio CPU/RAM load.

AppImage and release pipeline

• Build the AppImage on a sufficiently old compatible Linux base and audit its bundled/native dependencies.
• Produce artifact checksums and deterministic version metadata.
• Add Linux as a required artifact producer to Refactor LSDJ's cross-platform application foundation #107's single-publisher release orchestration; the release must not publish if the Linux producer fails.
• CI may compile and run platform-independent tests on hosted runners, but packaged audio/MIDI/NVIDIA qualification remains an explicit real-hardware gate.

Documentation

• Installation, executable bit/AppImage launch, NVIDIA driver floor, hardware requirements, audio/MIDI permissions, model storage, diagnostics, and known limitations.
• Clearly distinguish supported Ubuntu from community/untested distributions.

Required real-hardware checklist

Record exact OS, kernel, session type, CPU, RAM, GPU, VRAM, NVIDIA driver, audio device, MIDI controller, and runtime/model revisions.

• Run both MRT2 decks for at least 10 minutes in 25-frame / approximately 1-second mode.
• Run both armed decks for at least 10 minutes in 5-frame / approximately 200-ms mode.
• Require zero engine-reported underruns and capture p50/p95/p99 generation latency, queue depth, audio buffer settings, temperature, and throttling notes.
• Exercise FLX4 four-channel audio, all required controls, LED feedback, hotplug/reconnect, default-device change, and suspend/resume.
• Exercise Stable Audio music, SFX, audio-to-audio, continuation, inpainting, Small/Medium, LoRA, cancellation, and long-duration validation while decks remain active.
• Confirm normal and abnormal app exit leave no worker descendants.

Acceptance criteria

• A fresh supported Ubuntu 22.04+ x86_64/NVIDIA machine can install and run the AppImage without developer tools or system Python.
• Full MRT2 and Stable Audio feature parity is available under the documented backend limitations.
• The FLX4 audio/MIDI workflow passes the required real-hardware checklist on Wayland and Xorg.
• Both two-deck scheduling modes pass the 10-minute zero-engine-underrun gate on the published minimum hardware.
• Runtime/model installs, updates, interruptions, and corrupt downloads recover without losing the previous verified version.
• AppImage metadata, checksums, docs, and third-party notices are present.
• The central publisher refuses to publish when the required Linux artifact or its checks fail.

Out of scope

• ARM64.
• AMD or Intel GPU support.
• .deb, .rpm, Flatpak, or Snap packaging.
• JACK-specific support.
• TensorRT/custom CUDA Stable Audio acceleration.
• GPG signing beyond the release/checksum mechanism selected by the project.