Drive CPU registers and memory (1541 drive)

## Description

VICE exposes registers and memory for floppy drive CPUs (drive 8, 9, etc.) via different `memspace` values in the BMP protocol. The 1541 drive has its own 6502 CPU that can be debugged.

## Proposed implementation

- Add a second thread object in the trace for the drive CPU (e.g., `C64.Threads[1]` with display "1541 Drive 8")
- Query drive registers via `registers_get(memspace=MEMSPACE_DRIVE8)`
- Read drive memory via `memory_get(..., memspace=MEMSPACE_DRIVE8)`
- Add drive-specific memory regions (2KB RAM at $0000-$07FF, ROM, etc.)
- Update event handling to distinguish which CPU stopped

## Why this matters

Useful for debugging fast loaders, copy protection routines, and custom drive code — a niche but important C64 development area.