QEMU: Rust runner parity + /tmp relay units

[emilf]

This PR moves the QEMU workflow toward machine-control + human ergonomics.

Highlights

• Adds/extends (Rust runner) to cover key conveniences:
  • build-before-run (cargo build -Z build-std=core,alloc -Z json-target-spec --package theseus-bootloader --target x86_64-unknown-uefi-dwarf.json --release ) by default; disable with
  • support
  • success-marker detection (defaults to: ) to force exit code 0
• Adds systemd user relay units under for stable endpoints:
  • monitor PTY: +
  • serial PTY: +
  • debugcon PTY: +
  • QMP relay: →
• Adds installer helper: Installed qemu_serial_relay.service
  Installed qemu_monitor_relay.service
  Installed qemu_debugcon_relay.service
  Installed qemu_qmp_relay.service
• Adds convenience flag: (enables the standard /tmp relay paths in one go)
• Documentation updates:

How to try

1. Install relays:
   • Installed qemu_serial_relay.service
     Installed qemu_monitor_relay.service
     Installed qemu_debugcon_relay.service
     Installed qemu_qmp_relay.service
2. Run QEMU with relays:
   • cargo build -Z build-std=core,alloc -Z json-target-spec --package theseus-bootloader --target x86_64-unknown-uefi-dwarf.json --release

3. Attach terminals:

Notes

• sockets remain opt-in; provides a minimal-keystroke path for the common setup.
• This is intentionally scoped to QEMU invocation + endpoints; higher-level automation (tmux/QMP control) comes next.

[emilf]

Added runner parity for the startQemu.sh log-nerfing change: theseus-qemu now keeps headless QEMU debug output high-signal by default (-d guest_errors), and exposes --qemu-debug-flags [flags] + --no-qemu-debug to opt in/out. Also updated docs/qemu-runner.md.