KDE: add bookworm Wayland gate and release-aware SDDM session probe

Bookworm's plasma-workspace ships only the X11 session; the Wayland
session (plasmawayland.desktop) lives in the separate
plasma-workspace-wayland package (a Recommends of plasma-desktop, now
listed explicitly for the same determinism as the explicit sddm). The
package does not exist on trixie, where plasma-workspace integrates
Wayland natively, so naming it off-bookworm would fail the apt
transaction: resolve it once per installer with _kde_wayland_pkg() and
build the package list in a conditional array so neither the pre-apply
summary nor the apt command ever shows an empty argument. Also correct
the header comment: bookworm still defaults SDDM to the X11 session,
while trixie makes Wayland primary (plasma.desktop in
wayland-sessions) with plasmax11.desktop as the X11 fallback.

Replace the hard-coded SDDM autologin session cascade with an
existence-based ordered probe (_sddm_session_candidates) that prefers
Wayland across releases, keeps the LXQt sessions last for when the
module starts offering LXQt, and reads its session roots from
SESSION_DIR_WAYLAND / SESSION_DIR_X11 for hermetic testing.

Extend tests/desktops.bats with 13 offline tests: gate helper output on
bookworm/trixie, captured apt commands for both installers, summary
content (no empty package line), and the ordered session probe against
fixture trees.

modules/desktop_display.sh

@@ -299,6 +299,29 @@ configure_gdm3() {
     done
 }
 
+# Ordered session candidates for SDDM autologin:
+# Wayland first, then X11, per release. Existence
+# decides (no codename gates). The LXQt sessions
+# stay last for when the module starts offering LXQt.
+_sddm_session_candidates() {
+    local wayland_dir="${SESSION_DIR_WAYLAND:-/usr/share/wayland-sessions}"
+    local x11_dir="${SESSION_DIR_X11:-/usr/share/xsessions}"
+    local candidate
+    for candidate in \
+        "$wayland_dir/plasmawayland.desktop" \
+        "$wayland_dir/plasma.desktop" \
+        "$x11_dir/plasma.desktop" \
+        "$x11_dir/plasmax11.desktop" \
+        "$wayland_dir/lxqt-wayland.desktop" \
+        "$x11_dir/lxqt.desktop"; do
+        if [ -f "$candidate" ]; then
+            basename "$candidate" .desktop
+            return 0
+        fi
+    done
+    return 0
+}
+
 configure_sddm() {
     while true; do
         local choice
@@ -317,19 +340,11 @@ configure_sddm() {
             ;;
         2)
             local sddm_session=""
+            sddm_session=$(_sddm_session_candidates)
             local sddm_user="${SUDO_USER:-}"
             if [ -z "$sddm_user" ] || [ "$sddm_user" = "root" ]; then
                 sddm_user=$(awk -F: '$3>=1000 && $3<65534 {print $1}' /etc/passwd | head -1)
             fi
-            if [ -f /usr/share/wayland-sessions/plasmawayland.desktop ]; then
-                sddm_session="plasmawayland"
-            elif [ -f /usr/share/wayland-sessions/lxqt-wayland.desktop ]; then
-                sddm_session="lxqt-wayland"
-            elif [ -f /usr/share/xsessions/plasma.desktop ]; then
-                sddm_session="plasma"
-            elif [ -f /usr/share/xsessions/lxqt.desktop ]; then
-                sddm_session="lxqt"
-            fi
             sudo mkdir -p /etc/sddm.conf.d
             cat <<EOF | sudo tee /etc/sddm.conf.d/autologin.conf >/dev/null
 [Autologin]

modules/desktops/desktop_kde.sh

@@ -1,10 +1,32 @@
 #!/usr/bin/env bash
 # desktop_kde.sh — KDE Plasma desktop installer (SP2)
 # Full: kde-standard + sddm | Minimal: kde-plasma-desktop + sddm
-# Bookworm ships Plasma 5.27.5; Wayland is the default
-# session since Debian 12.
+# bookworm: Plasma 5.27.5 / KF5 / Qt5; SDDM defaults to
+# X11 (plasma.desktop in xsessions); Wayland is optional
+# via plasma-workspace-wayland (plasmawayland.desktop).
+# trixie: Plasma 6.3.6 / KF6 / Qt6; Wayland is the
+# primary session (plasma.desktop in wayland-sessions),
+# X11 the alternative (plasmax11.desktop). sddm is only
+# Recommends in the meta-kde chain, so the explicit sddm
+# in both routes is what guarantees the DM (and
+# xserver-xorg/xauth, hard deps of sddm in both releases).
 # License GPL v3
 
+# plasma-workspace-wayland exists only on bookworm:
+# bookworm's plasma-workspace ships the X11 session only,
+# and the Wayland session (plasmawayland.desktop) lives
+# in this separate package (a Recommends of plasma-desktop,
+# listed explicitly for the same determinism as the
+# explicit sddm). trixie integrates Wayland into
+# plasma-workspace itself and the package does not exist
+# there (packages.debian.org/trixie/plasma-workspace-wayland),
+# so naming it off-bookworm would fail the apt transaction.
+_kde_wayland_pkg() {
+    case "${DEBIAN_CODENAME:-}" in
+    bookworm) printf '%s\n' "plasma-workspace-wayland" ;;
+    *)        printf '%s\n' "" ;;
+    esac
+}
 kde_menu() {
     while true; do
         local -a kd_items=()
@@ -27,8 +49,15 @@ kde_menu() {
 
 _install_kde_full() {
     # Pre-apply summary: confirm the selection before any
-    # debconf or apt transaction starts.
-    _confirm_install_list "KDE Plasma Full" kde-standard sddm || return
+    # debconf or apt transaction starts. The Wayland
+    # session package is resolved once (bookworm only;
+    # the package does not exist on trixie) so the
+    # summary lists exactly what apt will install.
+    local wayland_pkg
+    wayland_pkg=$(_kde_wayland_pkg)
+    local -a pkgs=(kde-standard sddm)
+    [ -n "$wayland_pkg" ] && pkgs+=("$wayland_pkg")
+    _confirm_install_list "KDE Plasma Full" "${pkgs[@]}" || return
 
     echo -e "${GREEN}Installing KDE Plasma (Full)...${NC}"
 
@@ -42,7 +71,7 @@ _install_kde_full() {
     fi
     echo "${target_dm} shared/default-x-display-manager select ${target_dm}" | sudo debconf-set-selections
     _ensure_apt_updated
-    _run_cmd "KDE Full" "sudo apt install -y kde-standard sddm" \
+    _run_cmd "KDE Full" "sudo apt install -y ${pkgs[*]}" \
         "Installing KDE Plasma (Full Meta Package) + SDDM..."
     if [ "$target_dm" = "sddm" ]; then
         sudo systemctl enable sddm
@@ -56,8 +85,15 @@ _install_kde_full() {
 
 _install_kde_minimal() {
     # Pre-apply summary: confirm the selection before any
-    # debconf or apt transaction starts.
-    _confirm_install_list "KDE Plasma Minimal" kde-plasma-desktop sddm || return
+    # debconf or apt transaction starts. The Wayland
+    # session package is resolved once (bookworm only;
+    # the package does not exist on trixie) so the
+    # summary lists exactly what apt will install.
+    local wayland_pkg
+    wayland_pkg=$(_kde_wayland_pkg)
+    local -a pkgs=(kde-plasma-desktop sddm)
+    [ -n "$wayland_pkg" ] && pkgs+=("$wayland_pkg")
+    _confirm_install_list "KDE Plasma Minimal" "${pkgs[@]}" || return
 
     echo -e "${GREEN}Installing KDE Plasma Desktop (Minimal)...${NC}"
 
@@ -71,7 +107,7 @@ _install_kde_minimal() {
     fi
     echo "${target_dm} shared/default-x-display-manager select ${target_dm}" | sudo debconf-set-selections
     _ensure_apt_updated
-    _run_cmd "KDE Minimal" "sudo apt install -y kde-plasma-desktop sddm" \
+    _run_cmd "KDE Minimal" "sudo apt install -y ${pkgs[*]}" \
         "Installing KDE Plasma Desktop (Minimal) + SDDM..."
     if [ "$target_dm" = "sddm" ]; then
         sudo systemctl enable sddm