EnergyRouter's gate sharpness was a single fixed temperature shared by
every expert, with no way for an expert to independently learn how much
of the energy axis it covers. Adds two mutually exclusive, default-off
modes: learn_width (per-expert learnable width) and learn_temperature
(single learnable shared scalar), both bounded via a sigmoid
interpolation warm-started to reproduce today's fixed-temperature gate
exactly at init, to compare against each other without risking the
unbounded-width collapse failure mode. Also promotes gate_stats's
entropy into a generic, optional Router.entropy_loss (lambda_entropy) as
a secondary guard against all experts' widths co-inflating together.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
lars