Add typed CPU selection for QEMU gdbstub sessions

## What to build

Add typed CPU/thread selection for QEMU gdbstub DebugSessions. Multi-vCPU kernel debugging needs a safe way to inspect a selected CPU register and stack context without relying on implicit gdb thread state.

## Acceptance criteria

- [ ] A contributor can list gdbstub-exposed CPUs or threads for a live DebugSession.
- [ ] A contributor can select one CPU or thread for subsequent read-registers and backtrace operations.
- [ ] The response reports the current selected CPU or thread in structured data.
- [ ] Invalid or stale CPU/thread identifiers return categorized failures.
- [ ] Tests cover CPU listing, selection, register read after selection, invalid identifier, and malformed MI output.

## Blocked by

None - can start immediately.

Start by locating the DebugSession implementation and the existing read-registers, backtrace, and QEMU gdbstub MI handling. Trace how CPU or thread state is currently represented, then use the existing test structure to cover listing, selection, structured current-state reporting, invalid identifiers, and malformed MI output.

Written by the indexing model from the issue text.