Support multiple VICE machines (VIC-20, C128, PET, Plus/4)

## Description

VICE emulates multiple Commodore machines beyond the C64: VIC-20, C128 (with Z80 + 8502), PET, Plus/4, and CBM-II. The current plugin is hardcoded to C64/6510.

## Proposed implementation

- Make `arch.py` configurable per machine (language ID, memory map, register set)
- Add a launcher parameter to select the target machine
- For C128: support dual-CPU (Z80 + 8502) as two threads
- Use VICE's `BANKS_AVAILABLE` to auto-detect the machine's memory layout
- Different machines may need different Ghidra language IDs (e.g., Z80 for C128's Z80 mode)

## Why this matters

Expands the plugin's audience beyond C64 developers, though C64 is by far the most common target.