Depend on ttsim #141 (gep10-annotate-mettsim)

Repoint the ttsim-backend pin to the mettsim worked-example branch, which
sits on top of the merged per-system-unit-registries work and carries the
Python 3.11 mappingproxy import fix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: MImmesberger