feat: HDMI refresh boost per display, with a Saved displays screen

Rates are saved per display (EDID ID) and loaded by the hotplug script
for whichever display is connected. The item is on only while the
connected display is boosted; unticking removes just that one. The app
lists the saved displays and removes any of them (hdmi-forget).

AGENTS.md

@@ -186,13 +186,16 @@ gamescope and the Plasma desktop. Primary target: the Valve Steam Machine
   <output>=<w>x<h>:<rates>`; without `--hdmi` the backend refuses it. 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 override is never on the kernel command
-  line (that applied it to any display on the port): `steamify-edid-hotplug`
-  (boot unit + udev drm hotplug rule) loads it only while the display whose
-  DDC ID is in `/etc/steamify/hdmi-edid.conf` is connected, else resets.
-  It records the state in `/run/steamify-edid` before its own
-  `trigger_hotplug`, whose event runs it again. Pre-2.1.0 command-line
-  setups are removed by `hdmi_remove_boot_param`.
+  replaces the kernel's copy. EDIDs are saved per display
+  (`steamify-<id>.bin`, id = EDID bytes 8-17, listed in
+  `/etc/steamify/hdmi-edid.conf`), never on the kernel command line (that
+  applied to any display on the port): `steamify-edid-hotplug` (boot unit +
+  udev drm hotplug rule) loads the connected display's file, else resets. It
+  records what's loaded in `/run/steamify-edid` before its own
+  `trigger_hotplug`, whose event runs it again. `hdmi_status` is on only
+  while the connected display is boosted; disable forgets only that display
+  (`kpin_disable` forgets all). The app manages the list (`hdmi-forget`,
+  `hdmiDisplays` in status). Pre-2.1.0 setups are moved by `hdmi_migrate`.
 - Steam Machine CEC driver (`cec_driver_enable`, `lib/cec.sh`): mainline
   `cros_ec_cec` lacks Fremont, so HDMI-CEC builds Valve's copy (evlaV
   `linux-integration`, pinned commit + SHA-256) with DKMS for every kernel.