Add schematic-symbol capability to cells

[thanojo]

Cells can declare schematic_function= on @gf.cell to carry symbol/port metadata for schematic-capable tooling. This PDK doesn't have it yet.

Reference implementation: cspdk.

• Land the schematic factory in a dedicated _schematic.py module (see cspdk's per-band _schematic.py files), not scattered across individual cell files.
• Add a _schematic_tracking.md noting any symbol names that don't yet have a matching icon, so gaps stay visible instead of silently falling back to a generic box.