Fix display backlight on Dell XPS OLED Panther Lake laptops

[brian]

Problem

On the Dell XPS 14 (DA14260) with the LG OLED panel, the brightness keys move the OSD but the panel never changes. intel_backlight sysfs writes succeed and read back, so the Omarchy side is fine; the panel just isn't listening to that knob. The xe driver takes the backlight type from the VBT (PWM) while the OLED panel wants DPCD AUX. The kernel says so at boot:

xe 0000:00:02.0: [drm] [CONNECTOR:512:eDP-1] Panel is missing HDR static metadata.
Possible support for Intel HDR backlight interface is not used.
If your backlight controls don't work try booting with i915.enable_dpcd_backlight=3.

This is the same failure install/hardware/asus/fix-asus-ptl-display-backlight.sh already fixes for the ExpertBook B9406 and Zenbook UX5406AA (#5435, #5620), and the same one #10463 reports on the Lenovo Yoga Pro 7. The ASUS leaf is gated to those two models, so it never runs on a Dell. omarchy-hw-dell-xps-oled already matches this machine, but nothing uses it since the PSR workaround was retired in a70b05e.

Solution

• install/hardware/dell-xps-oled-display-backlight.sh: writes /etc/limine-entry-tool.d/dell-xps-oled-display-backlight.conf with xe.enable_dpcd_backlight=1, gated on omarchy-hw-dell-xps-oled. Mirrors the ASUS leaf; the drop-in path takes an OMARCHY_DELL_XPS_OLED_BACKLIGHT_CONF override for tests, like fix-nouveau-cursor.sh and snapper.sh do.
• install/hardware/all.sh: runs it next to the other Dell leaf.
• migrations/1788886195.sh: for existing installs, sources the leaf, runs limine-mkinitcpio, and sets reboot-required. Skips cleanly on machines without limine-mkinitcpio, since the drop-in does nothing without it. No completion marker: a second user running it before the reboot repeats an identical rebuild, which is harmless.
• test/shell.d/dell-xps-oled-display-backlight-test.sh: covers the all.sh wiring, the leaf writing and no-oping, idempotence, and the migration's rebuild, reboot flag, no-op, and missing-tool paths.

Value 1 lets the driver pick between the Intel HDR and VESA AUX interfaces, matching the ASUS fix. The kernel's own suggestion of 3 forces the Intel HDR interface; 1 was enough here.

Interaction with #10549

That PR tightens omarchy-hw-dell-xps-oled to the 2880x1800 and 3200x2000 OLED modes because the EDID manufacturer check alone also matches the 1920x1200 LG IPS SKU. This leaf inherits whichever matcher lands. With the current matcher the drop-in would also be written on an IPS XPS; at value 1 the driver only takes the AUX path when the panel advertises it, so the IPS panel should keep working over PWM, but I have no IPS unit to confirm that on. I'd rather not duplicate the mode check here; the matcher is the right place for it.

Testing

• Applied the same drop-in by hand on my XPS 14 DA14260 (Omarchy 4.0.3rc3-1, linux-ptl 7.2.3), rebuilt with limine-update, rebooted. /proc/cmdline carries the parameter and F7/F8 now change the panel.
• ./test/all: 234 of 237 shell test files pass, including the new one. The three that fail here fail on quattro too and do not touch anything in this diff: snapper-test.sh wants an omarchy-iso checkout, locate-test.sh reads /usr/bin/updatedb as text and hits a binary on this machine, and config-test.sh wanted an omarchy-pkgs checkout and passes once one is present.
• Mutation-checked the test: dropping the hardware gate from the leaf, or the limine-mkinitcpio guard from the migration, each fails the corresponding assertion.

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