feat: HDMI refresh boost (Steam Machine, pinned kernel)

The pinned 7.1.6 kernel doesn't read the extra EDID block (HDMI Forum
EEODB) where monitors list their fast modes, and does no HDMI 2.1, so HDMI
displays often stay at 60 Hz. The new menu item reads the display's EDID
over DDC, calculates the highest rate that fits HDMI 2.0 at the desktop
resolution (rounded down to ten, plus the hundred below as a safe option),
tests each step live with a confirmation, and makes the confirmed ones
permanent through drm.edid_firmware (Limine, sdboot-manage or GRUB).

AGENTS.md

@@ -171,6 +171,14 @@ gamescope and the Plasma desktop. Primary target: the Valve Steam Machine
   The start script closes its window after a 10-second countdown on success
   and waits for Enter after an error; it uses `pipefail`, or a failed
   download would run an empty script and count as success.
+- HDMI refresh boost (`hdmi`, `lib/hdmi-refresh.sh`): only on Fremont with
+  the pinned kernel (stays visible while on). It needs someone at the
+  screen: the graphical app's backend (`BACKEND=true`) must refuse it, and every step needs a "y" within 15 s
+  (`WIZARD_HDMI_CONFIRM_SECONDS` for scripted tests). debugfs is root-only
+  (glob it under sudo), and `edid_override` takes exactly `reset` with no
+  newline. Build the EDID from the DDC read, not from sysfs: a live override
+  replaces the kernel's copy. The kernel command-line value holds a path, so
+  `sed` uses `|` as its delimiter.
 - `Relogin=true` means a gamescope that fails to start is relaunched in a
   tight loop; keep that in mind when changing session handling.