v0.21.2 — designators, told to agents

Volt numbers every part it builds a board from — R, C, L, D for any diode, Q for any transistor, U for any IC — per letter, in creation order, and the board calls a part what the schematic calls it.

circuit.json now says which letter each type takes, that numbering is automatic, and that data.name is the override to leave alone unless the user asked for a specific designator.