feat(tools): commit the VM test harness + one-command installer regression

Promotes the throwaway harness that verified the installer into tools/:
- tools/vm/qmp.py        QMP keystroke injection / typing / quit
- tools/vm/vncshot.py    GL-safe screenshots via VNC readback (QMP
                         screendump shows "no surface" with virtio-vga-gl)
- tools/vm/gap-analysis.py  drv-graph diff that converged the offline
                         pin set; run it when offline installs build
                         from source
- tools/test-install.sh  the full offline regression: build ISO, boot
                         offline, unattended LUKS+swap install via a
                         config disk (typed long commands drop keys),
                         wait for poweroff, boot the installed disk,
                         screenshot the first boot for visual verdict

docs/TESTING.md §4 now points at the script.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -65,7 +65,10 @@ Flat on purpose. Two module trees, one options file each, no hidden layers.
 ├── docs/TESTING.md                # how to verify changes (incl. AI-agent rules)
 └── tools/                         # maintainer-only
     ├── import-palettes.py         #   converts old-distro themes → JSON + assets
-    └── test-live-iso.sh           #   build the ISO + boot it in QEMU
+    ├── test-live-iso.sh           #   build the ISO + boot it in QEMU
+    ├── test-install.sh            #   full offline-install regression in QEMU
+    └── vm/                        #   headless-VM helpers (QMP keys, VNC shots,
+                                   #   offline-pin gap analysis)
 ```
 
 **Rule of thumb:** `modules/` is the distro (reusable, no machine specifics),