Add zero-copy GPU encoding pipeline

- New GPUHLSOutput class for direct GPU-to-NVENC encoding
- RGB→NV12 conversion via CUDA kernel (no CPU transfer)
- Uses PyNvVideoCodec for zero-copy GPU encoding
- ~220fps vs ~4fps with CPU pipe approach
- Automatically used when PyNvVideoCodec is available

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile.gpu

@@ -66,6 +66,9 @@ RUN pip install --no-cache-dir -r requirements.txt
 # Install GPU-specific dependencies (CuPy for CUDA 12.x)
 RUN pip install --no-cache-dir cupy-cuda12x
 
+# Install PyNvVideoCodec for zero-copy GPU encoding
+RUN pip install --no-cache-dir PyNvVideoCodec
+
 # Copy decord from builder stage
 COPY --from=builder /decord-install /usr/local/lib/python3.11/dist-packages/
 COPY --from=builder /tmp/decord/build/libdecord.so /usr/local/lib/