qtvcp probe help: load first help page lazily

[grandixximo]

Problem

qtdragon (and any qtvcp screen using the basic-probe or versa-probe widgets) can hang or crash at startup on a headless / no-GPU Qt platform (QT_QPA_PLATFORM=offscreen, no GPU). I hit this consistently on a CI runner and reproduced it in a plain ubuntu:24.04 container with no GPU: qtvcp never finishes coming up, so the GUI fails to start.

Root cause

The basic-probe and versa-probe help dialogs preload their first help page during widget construction (buildWidget, called from hal_init). Since the help loader gained smooth image pre-scaling (_set_scaled_html, QImage with SmoothTransformation), that preload now builds and scales the raster help diagrams at startup.

That scaling on the startup path is what breaks headless: in my no-GPU container qtvcp hangs there; on the CI runner it segfaults. I confirmed it by bisecting the qtvcp Python in the container:

• qtdragon ui-smoke as-is: fail (hang in container, segfault on CI)
• revert versa_probe.py + basic_probe.py to before the smooth-scale change: pass
• keep the files but stub _set_scaled_html to a plain setHtml (kill only the scaling): pass

So the regression is specifically the image scaling running during construction. It is also wasted work whenever the operator never opens the help dialog.

Fix

Defer the first-page load from buildWidget to the first showDialog() call, in both basic_probe.py and versa_probe.py. The help looks and behaves identically when opened; the image scaling just no longer runs on the startup path.

Testing

In a no-GPU ubuntu:24.04 container (RIP build, offscreen platform), running the qtdragon GUI smoke test:

• before: qtdragon fails to come up
• after: qtdragon UI_SMOKE_OK; touchy smoke unchanged (control)