full: SDDM-only greeter — gate off competing display managers

Operator confirmed "SDDM is the only greeter". Disable the two competing
display entry points that would contend with SDDM for the single-owner
GPU/card0: 20_greeter (redbear-greeterd, an alternative greeter) and 20_display
(redbear-session-launch, a boot-time auto-KDE-session that bypasses the
greeter). Gated reversibly via condition_path_exists on a sentinel
(/etc/redbear/enable-legacy-greeter) the image does not install — definitions
preserved, re-enable with a single touch. 21_sddm remains ungated. Post-login
session launch is SDDMs job via SessionDir=/usr/share/wayland-sessions.

config/redbear-full.toml

@@ -424,6 +424,12 @@ path = "/etc/init.d/20_display.service"
 data = """
 [unit]
 description = "KDE session assembly helper"
+# Disabled for the SDDM-only greeter path: this auto-starts a KDE session at
+# boot (bypassing the greeter) and would contend for the single-owner
+# GPU/card0 with SDDM. Gated on a sentinel that the image does not install;
+# `touch /etc/redbear/enable-legacy-greeter` re-enables it. Post-login session
+# launch is SDDM's responsibility (SessionDir=/usr/share/wayland-sessions).
+condition_path_exists = ["/etc/redbear/enable-legacy-greeter"]
 requires_weak = [
     "12_dbus.service",
     "13_redbear-sessiond.service",
@@ -443,6 +449,11 @@ path = "/etc/init.d/20_greeter.service"
 data = """
 [unit]
 description = "Red Bear greeter service"
+# Disabled for the SDDM-only greeter path (operator: "SDDM is the only
+# greeter"). redbear-greeterd is an alternative greeter that would contend
+# with SDDM for the GPU/card0. Gated on a sentinel the image does not install;
+# `touch /etc/redbear/enable-legacy-greeter` re-enables it.
+condition_path_exists = ["/etc/redbear/enable-legacy-greeter"]
 requires_weak = [
     "00_driver-manager.service",
     "12_dbus.service",