Kicksecure ISO live amnesic (USER Session) boot fails on novacustom-v540tu; works on x230 and nv4x_adl

A pull request for this has already been merged.

• #2186 by @tlaurion — merged

## Before filing a bug report

Follow-up to #2008 and #2186. #2186 merged in master.

## Hardware details

- Board: **novacustom-v540tu** (failing). Working: x230 (EOL_x230-maximized), novacustom-nv4x_adl.
- GPU: iGPU-only (Intel Meteor Lake, device ID 7d55).
- Heads/Dasharo: `Dasharo Heads-v0.2.1-3144-g7850f7a` (DMI BIOS string), vendor-installed by Novacustom.
- ROM: vendor (Dasharo) image, maximized.

## Please describe the problem

**Describe the bug**

#2186 ("initrd: boot dd-written hybrid ISOs in mount-usb and usb-autoboot") fixes #2008 ("kicksecure iso doesn't boot under Heads") on x230 and novacustom-nv4x_adl. On novacustom-v540tu the Kicksecure ISO fails to boot **only in live-amnesic mode** ("LIVE Mode | USER Session | disposable use"): the machine auto-reboots before/at the LXQt desktop.

**The failure is specific to the live-amnesic USER session.** Non-live-amnesic modes boot fine on the same machine: "LIVE Mode | SYSMAINT Session" (sysmode) boots to console, and the other grub entries are unaffected.

**To Reproduce**

1. Boot the Kicksecure 18 live ISO on novacustom-v540tu via Heads (dd-written hybrid ISO path from #2186).
2. Select "LIVE Mode | USER Session | disposable use".
3. The machine auto-reboots before the desktop.
4. Select "LIVE Mode | SYSMAINT Session" instead: boots to console fine. Both entries work on x230 and novacustom-nv4x_adl.

**Expected behavior**

Live amnesic USER session boots to the LXQt desktop with autologin, as on x230 and nv4x_adl.

**Additional context**

Full sysmode capture (the mode that was successfully logged, since live-amnesic reboots before a capture is possible): https://gist.github.com/tlaurion/f9d3070cd0126a190fe52eec8732c302 (`kicksecure_sysmode_v540tu.log`).

The two modes pass an identical kernel cmdline except the session target (`graphical.target` + autologin vs `systemd.unit=sysmaint-boot.target` + `noautologin`), so the failure is at graphical/display bring-up, not the overlay. The v540tu is the only machine whose i915 driver does not initialize cleanly. Relevant lines from the gist above:

- L1926: `i915 0000:00:02.0: [drm] Found METEORLAKE (device ID 7d55) display version 14.00`
- L2360: `i915 0000:00:02.0: [drm] [ENCODER:249:DDI B/PHY B] failed to retrieve link info, disabling eDP`
- L2393-L2437: panel power-sequencing WARN + Call Trace:
- L2394: `i915 0000:00:02.0: [drm] drm_WARN_ON(intel_dp->pps.vdd_wakeref)`
- L2395: `WARNING: CPU: 8 PID: 1012 at drivers/gpu/drm/i915/display/intel_pps.c:755 intel_pps_vdd_on_unlocked+0x32b/0x340 [i915]`
- L2402: `RIP: 0010:intel_pps_vdd_on_unlocked+0x32b/0x340 [i915]`
- L2414: `Call Trace:` ... `drm_dp_dpcd_probe` / `intel_dp_detect` / `intel_dp_set_edid`
- L3565: `i915 0000:00:02.0: [drm] *ERROR* GT1: GSC proxy component didn't bind within the expected timeout`
- L3566: `i915 0000:00:02.0: [drm] *ERROR* GT1: GSC proxy handler failed to init`

x230 and nv4x_adl show clean i915 init (zero kernel WARNINGs).

Also noted: the v540tu final ISO cmdline retains `intel_iommu=on` while x230/nv4x_adl strip it, traced to a per-machine `config.user` override (config-gui "Debug/Output" menu) rather than a board-config difference. Unlikely the cause (no DMAR faults on v540tu), but worth checking.

This looks like an upstream i915/Meteor Lake display issue surfaced by the graphical autologin session. Next steps: reproduce in plain Debian trixie, and capture a live-amnesic boot over the Heads serial console to confirm the failure at graphical.target.

Start with the linked sysmode capture and compare the live-amnesic USER session with the sysmaint path, focusing on graphical.target, autologin, and the i915 warnings. Reproduce on plain Debian trixie and capture the failing boot over the Heads serial console; done means the graphical-session failure is confirmed and its cause is isolated, including whether intel_iommu=on is relevant.

Written by the indexing model from the issue text.