DS Netplay core: port GPL DetMP and deterministic mirrored melonDS to native Linux #163

Parent: #162. Implement the emulator-side mechanism, not social/network UI.

Technical target

Port the useful ideas/source from GPL-licensed WatermelonDS Emufii Edition into a reproducible, TrainerOS-owned Linux melonDS build. Prefer a maintainable small patch set on a selected pinned upstream melonDS revision. Do not package the Android WatermelonDS APK, Kotlin UI, Android logging/properties/JNI or a parallel all-in-one launcher.

Start from exact inspected source

• WatermelonDS Android netplay @ 9b603e53d4d098a0fcee3401ffa5e9c9992f01a3 — input frames, ENet handshake, per-peer mirrors, adaptive lag, hashes/checks.
• Modified melonDS deterministic local wireless core @ 85fdb7596011a3d0e679a6f1f88c312e77ff4cc5 — emulated clock/time ordering and deterministic packet/command/reply exchange.
• Check all related upstream core edits and patch ancestry: src/net/DetMP.*, MPInterface.*, NDS scheduling/frame hooks, Wi-Fi calls, deterministic serialization/savestate/JIT normalization, mirror lifecycle and renderer. One NetplayAndroid.cpp copy is insufficient.
• Compare with current upstream melonDS and any upstreamed implementation before patching; prefer maintained upstream solution where demonstrably equivalent, without erasing provenance.

Implementation

• Add a build target/profile and versioned patch in TrainerOS's managed emulator pipeline similar to the existing Dolphin netplay bridge; document reproducible source/toolchain/dependencies, hashes and ABI/platform support.
• Use a narrow platform-neutral/netplay core API; replace Android __android_log_print, system properties, JNI/Kotlin, per-APK preferences and ad hoc user host/join dialog with Linux/core settings driven by trusted local TrainerOS integration.
• Each physical client runs one local rendered DS plus one non-presented mirror for each other player. Local wireless interactions happen between co-located instances in emulated deterministic time; the network carries controlled inputs/synchronization, not raw timed DS Wi-Fi over a generic VPN.
• Deterministically order wireless transmissions (frame/cycle timestamps, sender ID and sequence) and ensure JIT/core settings do not introduce divergence. Synchronize starting conditions and app state under DS Netplay security: lawful local BIOS, private saves and bounded mirror synchronization #164 before allowing a live lobby.
• Adapt host-selected network lag to measured RTT/jitter rather than freeze a magic 4-frame default; track user-visible latency and failure modes.
• Reuse diagnostic hash/checkpoint/desync comparison where appropriate, but never log raw memory, saves, BIOS, player identifiers or credentials.
• Mirror-only rendering optimizations must be separately proven safe with GPU display captures (upstream Mario Kart issue is relevant), not blindly applied to Linux GPU paths.
• Ensure ordinary single-player melonDS, local LAN mode and WFC game networking are unchanged when DS netplay is disabled.

Licensing

TrainerOS code is GPL-3.0-or-later (#115). Upstream melonDS/WatermelonDS changes are GPL-compatible. Preserve original copyright headers, exact source commit IDs, changed-file notices and corresponding GPL source/patch distribution. ENet MIT and zstd BSD-3-Clause notices/dependency versions must be retained. Keep Android-only code out of the native build.

Acceptance

• Reproducible Linux ARM64 build and x86-64 build where feasible; selected existing single-player DS games still launch/exit/save normally.
• Two co-located emulated DS exchange representative wireless frames deterministically under repeatable tests without any real network.
• Recorded per-frame inputs reproduce a deterministic session under matched build/settings; controlled desync is detected.
• Android-specific glue and unbounded raw network/savestate parsing are not carried over without adaptation.
• GPL/notices, sources/patches, release hashes and reproducible runtime identity are included under TrainerOS Software (Distribution: independent image and TrainerOS software release manifests and ownership #119/Runtimes: deploy a TrainerOS-owned emulator stack on SteamOS and Bazzite #121/TrainerOS Software: signed cross-distro updater independent of system image #154).
• Further privacy/session-state requirements in DS Netplay security: lawful local BIOS, private saves and bounded mirror synchronization #164 must pass before any internet play is advertised.