release: v1.20.1 — an idle die is a reading, not a missing one

Fixed

• The temperature source could claim it had no reading while it was visibly driving the
  animation. Caught by hand in the release's own menu walk: the dashboard showed
  Temperature: warming up... on one row and Speed Multiplier (auto: Temperature): 1.74x two rows
  below it, after minutes of correct readings. The cause is a distinction 1.20.0 failed to make. A
  powered-down core cluster answers the sensor read with 0 rather than declining it, and those get
  filtered out so the menu doesn't print -4 °C; but on a tick where every cluster happens to be
  parked, the filter emptied the whole set and the reader reported that as missing data. It
  isn't — it is a reading of idle. Only a total read failure (the SMC going away mid-run) is
  missing data, and the two are now separate: an all-parked tick reports an idle load and the menu
  says Temperature: ≤30 °C · every core cluster parked rather than inventing a measurement.

  Rare — it did not recur in 45 sampled ticks or three targeted attempts to reproduce it — and
  cosmetic when it fired, since the animation simply held its last speed. Recorded in
  docs/ROADMAP.md § Verification debt, because the parked branch cannot be reached on demand: the
  only hook that would force it is one that changes a decision, which this repo's testing rules bar.