feat: switch host from intel VirtualHeads to dummy driver for headless boot

- Add 20-dummy-headless.conf (Driver dummy, 1920x1080)
- Remove 20-intel-virtual.conf (intel driver required HDMI at boot)
- Update install.sh: installs dummy driver, saves EDID, updates GRUB
- Update README: document dummy driver + kernel EDID fallback approach
- hdmi-fallback.sh now detects DUMMY0 or VIRTUAL1

Rationale: intel VirtualHeads only worked when HDMI was connected at boot.
Dummy driver creates DUMMY0 unconditionally, working with or without cable.
Kernel EDID fallback (video=... drm.edid_firmware=...) is insurance if dummy fails.

config/x11-host/20-intel-virtual.conf.bak

@@ -0,0 +1,9 @@
+# BACKUP of original Intel VirtualHeads config
+# Disabled in favour of dummy headless config (20-dummy-headless.conf)
+# which works without HDMI connected at boot.
+#
+# Section "Device"
+#     Identifier "Intel Graphics"
+#     Driver "intel"
+#     Option "VirtualHeads" "1"
+# EndSection