Three fixes, found by pointing the tool at the laptop's built-in card when PLN
moved upstairs and played UMC-less out of the aux jack into a Pioneer amp.

1. The rate and STALL checks never ran. They compared consecutive polls and
   required dt > 0.5 s, but the default poll interval is 0.25 s, so the
   condition was almost never true: the 900 s watch that diagnosed the UMC
   collected exactly TWO rate samples. The restart detector carried the entire
   investigation while the other half of the tool was decoration. Now measured
   against a reference sample held at least a second back, which also makes
   'RUNNING but hw_ptr frozen' a check that can actually fire. Verified: 11
   samples in 12 s, 47984/47996/48034 Hz.

2. It watched pcm0p unconditionally. Fine for a USB interface with one playback
   PCM; wrong for the SOF card, which has eight (headphones, speakers, mics,
   HDMI) and where which one is live depends on where the jack is. It would
   have reported perfect health about the speakers while the sound went out the
   headphone jack. Now prefers a RUNNING substream, re-resolved every poll, and
   says so when the live PCM moves.

3. nominal_rate read stream0, which is USB-audio-only, so it returned None on
   the built-in card — disabling the rate comparison on exactly the device PLN
   falls back to when the interface dies. Falls back to hw_params.

Also stops crying wolf: a stream that just restarted has a hw_ptr counting from
zero, so the first delta across the seam is meaningless. Measuring it anyway
printed 'RATE OFF - 0 Hz (-100%)' on every reconnect during the replug test — a
false alarm fired at the exact moment the operator most needs to trust the
tool.

The lesson is the one this repo keeps relearning: a check that never fires is
indistinguishable from a check that passes, and you only find it by running the
thing somewhere new.