GSoC 2026: RasterLayer Refactor: Tracking Issue

[Tejasv-Singh]

Tracking issue for the GSoC 2026 RasterLayer refactor project.
Architecture decisions are documented in #327.

Deliverables

• NumPy-backed storage backend : replace _initialize_cells() with
  dict[str, np.ndarray]
• Cell proxy : decouple Cell from mesa.Agent, dynamic band access
  via __getattr__/__setattr__
• Collection API : RasterCellCollection with do, shuffle_do,
  select
• SolaraViz pipeline : replace per-polygon renderer with single
  ImageOverlay

cc @wang-boyu @jackiekazil