feat: 1.6 is all our own code, no NirSoft fallback (#91)

[lippdev]

Summary

Removes every trace of the NirSoft tools from the app, as decided for 1.6:

• MultiMonitorTool and SoundVolumeView backends, the NativeAudio / NativeDisplays flags, the tool paths and the "tool missing" strings are gone.
• MonitorService uses a single DisplayBackend (Windows APIs); AudioService uses Core Audio only.
• Unused leftovers removed: the SoundVolumeView volume parser (and its test) and CsvReader.
• On startup the app deletes the MultiMonitorTool.exe / SoundVolumeView.exe that 1.5 extracted into tools.
• Manual checklist now compares with 1.5 instead of a flag; changelog under Unreleased.

Still not verified on hardware.

🤖 Generated with Claude Code

[pullfrog]

✅ No new issues found.

Reviewed changes This review covers the removal of NirSoft backends and the resulting unconditional use of the existing Windows-native display and audio implementations.

• Backend selection removed The NativeAudio and NativeDisplays flags, NirSoft process paths, and backend-selection code are removed, making Core Audio and Windows display APIs the sole implementations.
• Legacy tool cleanup Startup removes old NirSoft executables from the tools directory while retaining RTSS CLI setup.
• Display identity and restoration retained The native display backend continues to use the legacy layout format and maps saved monitor identities during restore.
• Documentation and cleanup The unused SoundVolumeView CSV parser and test are removed, while the manual checklist and Unreleased changelogs are updated for 1.5-to-1.6 upgrades.

  ^{｜ View workflow run ｜ Using GPT Luna ｜ 𝕏}