fix: discover_realsense() re-runs ~2s USB enumeration on every call

Summary

physicalai.capture.discover_all() is slow (>2s per call) because discover_realsense() creates and discards a pyrealsense2 context on every invocation. pyrealsense2 re-runs a full cold USB enumeration (~2s) whenever no rs.context() is alive in the process. Holding a single context alive for the process lifetime makes every discovery instant.

Steps to reproduce

physicalai/capture/cameras/realsense/_discover.py:

def discover_realsense() -> list[DeviceInfo]:
    ...
    ctx = rs_any.context()          # local context
    for i, dev in enumerate(ctx.query_devices()):
        ...
    return results                  # ctx is GC'd here → next call pays ~2s again

The context is local, so it's garbage-collected on return. pyrealsense2 tears down its USB enumeration state when the last context dies and re-pays the cold-enumeration cost (~2s) on the next context()/query_devices().

Logs and traceback

Environment

physicalai==0.1.1
macOS, pyrealsense2 installed, no RealSense device connected
Reproducible with zero cameras attached

Additional context

No response