Area-detector: BO struct objectives, ROI show/hide, 16M perf, CLI help

[pecomyint]

Summary

• perf: area-detector viewer stays responsive on large (16 MP) detectors — pyqtgraph autoDownsample, live autoscale throttled to ~2 s (histogram anti-shake fix kept), default plotting rate 14 Hz, and a per-process + system CPU readout.
• fix (Bayesian): BO objectives can read a field of a PVA struct PV (e.g. ManualROI:Stats) — the bridge now reports one scalar data_key per struct field instead of an invalid object dtype; a blank field on a struct fails with an actionable message.
• fix (ROI): manual ROIs honor the global "Show ROIs" toggle; geometry still persists.
• fix (CLI): removed the broken global help flags; DashPVA -h now points to DashPVA COMMAND --help.

Test

ruff clean; pytest 195 passed (unit + integration).