docs: close Phase B Passo 9 — dual-boot procedure (RUNBOOK §15)

RUNBOOK §15: dual-boot operation — Linux⇄Windows switch procedure, per-OS
CI-stack table (roots/runner/vmrun), current GRUB behaviour (DEFAULT=0,
TIMEOUT=0 hidden → boots straight to Linux), post-boot verification commands
for both OSes, and an optional GRUB tweak (left unapplied — boot-to-Linux is
intentional so the box returns as the Linux CI host after any unattended
reboot).

Checklist: Passo 9 done; fixed the stale global tracking table (Passi 6–9
were still [ ] despite being complete). Phase B is now fully closed (1–9 [x]).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/RUNBOOK.md

@@ -764,3 +764,87 @@ both hosts).
 > baselines whose IP-acquire is dominated by high-variance VMware-Tools polling
 > (σ 26–57 s); the 10-iteration static (§8/§11) and Linux (§10/§12) rows are the
 > reliable host-effect signal.
+
+---
+
+## 15. Dual-boot operation (Linux ⇄ Windows host)
+
+The CI machine is **dual-boot** on one piece of hardware: Linux Mint and
+Windows 11 never run at the same time. Each OS carries a full CI stack
+(orchestrator + runner + templates under its own root). Booting Linux brings
+up the Linux runner; booting Windows brings up the Windows runner. Templates
+and the `burnin-dummy` repo are kept at **template parity** across both roots.
+
+| | Linux host | Windows host |
+| --- | --- | --- |
+| CI root | `/var/lib/ci/` | `F:\CI\` |
+| Runner | `act-runner.service` (systemd) | `actions-runner` (Windows service / NSSM) |
+| Transport to guests | WinRM (Win guest) / SSH (Linux guest) | same |
+| vmrun | `/usr/bin/vmrun` | `C:\Program Files (x86)\VMware\VMware Workstation\vmrun.exe` |
+
+### Current GRUB behaviour
+
+`GRUB_DEFAULT=0`, `GRUB_TIMEOUT=0`, `GRUB_TIMEOUT_STYLE=hidden` — the machine
+boots **straight into Linux Mint with no menu**. Boot entries present:
+`Linux Mint 22.3 Cinnamon` (default) and
+`Windows Boot Manager (on /dev/nvme0n1p1)`.
+
+### Switch Linux → Windows
+
+With the hidden/zero-timeout GRUB, hold **Esc** (or **Shift**) during early
+boot to reveal the GRUB menu, then pick `Windows Boot Manager`. For a planned,
+unattended switch, set a one-shot next-boot target instead:
+
+```bash
+# Requires GRUB_DEFAULT=saved (see "Optional GRUB tweak" below).
+sudo grub-reboot "Windows Boot Manager (on /dev/nvme0n1p1)"
+sudo reboot
+```
+
+Before rebooting out of Linux: confirm no job is mid-flight
+(`sudo -u ci-runner ls /var/lib/ci/build-vms/` empty) and optionally pause the
+Linux runner in the Gitea UI so queued jobs wait for the Windows runner.
+
+### Switch Windows → Linux
+
+Reboot; with `GRUB_DEFAULT=0` the machine returns to Linux automatically (no
+key needed). If `grub-reboot` / `GRUB_DEFAULT=saved` is in use, Linux is the
+saved default after a normal Linux shutdown.
+
+### Post-boot verification (either OS)
+
+Linux:
+```bash
+systemctl is-active act-runner        # → active
+systemctl --failed                    # → 0 loaded units
+systemctl list-timers 'ci-*'          # all ci-* timers scheduled
+# then confirm the runner shows online in the Gitea UI (Admin → Runners)
+```
+
+Windows (PowerShell):
+```powershell
+Get-Service actions-runner            # Status Running
+& 'C:\Program Files (x86)\VMware\VMware Workstation\vmrun.exe' list   # vmrun OK
+# then confirm the runner shows online in the Gitea UI
+```
+
+### Optional GRUB tweak for everyday dual-boot
+
+To get a short pick menu and remember the last choice (handy if Windows is used
+often), edit `/etc/default/grub`:
+
+```bash
+GRUB_DEFAULT=saved
+GRUB_SAVEDEFAULT=true
+GRUB_TIMEOUT=5
+GRUB_TIMEOUT_STYLE=menu
+```
+
+```bash
+sudo update-grub
+```
+
+This is **optional** — the default boots-to-Linux behaviour is intentional so
+the machine comes back as the Linux CI host after any unattended reboot
+(power-loss, kernel update). Leave it as-is unless Windows becomes the daily
+driver.