Replaces the opaque variance/√12 formula with two explicit noise components:
sigma (default 0.05 °C) — white Gaussian noise, calibrated against the
20260628T184903 Sud-0010 log (detrended hold-phase tick-to-tick std ≈ 0.053 °C).
stirrer_sigma / stirrer_tau — optional AR(1) low-frequency component for
stirrer-induced fluctuations (off by default); steady-state std equals
stirrer_sigma, correlation time equals stirrer_tau ticks.
plant_factory.py updated to use sigma=0.05 instead of the old variance=0.01
(which gave std ≈ 0.003 °C — far too quiet).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>