Phase 7 Part B: Windows capture/processing backends

[nullEFFORT]

Context

Part A (#44) provides the platform-independent SDK with ABCs and mock backends. Part B implements the real Windows + NVIDIA GPU backends.

Classes to implement

File	Class	ABC	Library
capture/screen.py	MSSScreenCapture	ScreenCapture	mss
capture/audio.py	SoundDeviceCapture	AudioCapture	sounddevice
capture/video.py	NVENCVideoRecorder	VideoRecorder	ffmpeg/nvenc
processing/ocr.py	PaddleOCREngine	OCREngine	paddleocr
processing/stt.py	WhisperSTT	SpeechToText	faster-whisper
processing/diarize.py	NeMoDiarizer	SpeakerDiarizer	nemo_toolkit
hud/overlay.py	DearPyGuiOverlay	HUDOverlay	dearpygui

Requirements

• Windows 10/11 with NVIDIA GPU (RTX 3060+)
• CUDA 12.x + cuDNN 8.x
• Context handoff: client/CLAUDE.md + client/WINDOWS_BUILD.md

Acceptance criteria

• All 7 real backends implemented
• Existing 24 tests still pass
• End-to-end test on Windows with GPU
• Pipeline runs: python -m nightshade_client start --server HOST:PORT --api-key KEY --operator-id UUID --game "Name"