feat: Migrate Pillow simulator (sim_backend).

[nedseb]

Description

Move the Pillow-based display simulator from micropython-steami-tutorials.

Source

micropython-steami-tutorials/sim/sim_backend.py (195 lines)

Tasks

• Copy sim/sim_backend.py
• Adapt imports to reference steami_screen from micropython-steami-lib
• Verify SimBackend works standalone: python -c "from sim.sim_backend import SimBackend; ..."
• Add basic test

SimBackend API

• Implements the same interface as SSD1327Display/GC9A01Display (fill, pixel, text, line, etc.)
• Adds save(path, circular=True) for PNG output
• Font scaling to match MicroPython 8x8 pixel font weight
• Circular mask for round screen simulation

Parent

• feat: Scaffold repo and migrate simulation/validation tools. #1 — Migration umbrella issue