Restructure for bare Fedora: lib/ + modules/, DE selection restored

Splits setup.sh into lib/common.sh, lib/session.sh and seven modules
(3 DE, 4 TWM). Targets a bare Fedora 44+ install.

Fixes carried over from the first build:
- DEBUG_MODE was unbound under set -u, aborting the script at the version banner
- mangled 'echohttps://...' line left over from the starburst split
- set -e was missing entirely; --debug flag parsing restored
- unguarded 'clear' aborted the run without TERM
- $PRETTY_NAME unguarded under set -u
- modules were sourced, leaking shell options into the parent
- Flathub added twice (system-wide + per-user)
- no dnf5-plugins check before using 'dnf config-manager'
- no Fedora release gate
- /usr/share/xsessions assumed to exist when writing session entries

lib/session.sh

@@ -0,0 +1,91 @@
+# shellcheck shell=bash
+#------------------------------------------
+# lib/session.sh — display manager and graphical session wiring
+#
+# DM policy:
+#   * A DE module leaves its native DM alone (GNOME→gdm, Plasma→sddm, XFCE→lightdm).
+#   * A TWM module enforces SDDM, because SDDM is the only DM that presents both
+#     Wayland compositors and X11 sessions in one menu. One greeter, no surprises.
+#------------------------------------------
+
+if [[ -n "${_SB_SESSION_LOADED:-}" ]]; then
+    return 0
+fi
+_SB_SESSION_LOADED=1
+
+# Name of the active display manager ("sddm", "gdm", "lightdm"), or "" if none.
+detect_display_manager() {
+    local link=/etc/systemd/system/display-manager.service
+    if [[ -L "$link" ]]; then
+        basename "$(readlink "$link")" | sed 's/\.service$//'
+    else
+        echo ""
+    fi
+}
+
+enable_graphical_target() {
+    sudo systemctl set-default graphical.target
+}
+
+# Make $1 the display manager, disabling whatever is active now.
+switch_display_manager() {
+    local target="$1"
+    local current
+    current=$(detect_display_manager)
+
+    if [[ "$current" == "$target" ]]; then
+        ok "Display manager already $target."
+    else
+        if [[ -n "$current" ]]; then
+            info "Disabling current display manager: $current"
+            # disable may fail if the unit vanished; that is not fatal
+            sudo systemctl disable "$current" 2>/dev/null || true
+        fi
+        info "Enabling display manager: $target"
+        sudo systemctl enable "$target"
+    fi
+
+    enable_graphical_target
+}
+
+# Register a Wayland session so the DM can list it.
+# $1 = identifier, $2 = display name, $3 = command
+write_wayland_session() {
+    local id="$1" name="$2" exec_cmd="$3"
+    local file="/usr/share/wayland-sessions/${id}.desktop"
+    if [[ -f "$file" ]]; then
+        ok "Wayland session already registered: $id"
+        return 0
+    fi
+    info "Registering Wayland session: $id"
+    sudo mkdir -p "$(dirname "$file")"
+    printf '[Desktop Entry]\nName=%s\nComment=%s\nExec=%s\nType=Application\nDesktopNames=%s\n' \
+        "$name" "$name session" "$exec_cmd" "$id" | sudo tee "$file" >/dev/null
+    sudo chmod 644 "$file"
+}
+
+# Register an X11 session so the DM can list it.
+write_x11_session() {
+    local id="$1" name="$2" exec_cmd="$3"
+    local file="/usr/share/xsessions/${id}.desktop"
+    if [[ -f "$file" ]]; then
+        ok "X11 session already registered: $id"
+        return 0
+    fi
+    info "Registering X11 session: $id"
+    # A bare Fedora has no X session yet, so the directory may not exist.
+    sudo mkdir -p "$(dirname "$file")"
+    printf '[Desktop Entry]\nName=%s\nComment=%s\nExec=%s\nType=Application\n' \
+        "$name" "$name session" "$exec_cmd" | sudo tee "$file" >/dev/null
+    sudo chmod 644 "$file"
+}
+
+# Common login-time plumbing every Wayland compositor wants on a bare install.
+install_session_prereqs() {
+    install_packages \
+        xdg-desktop-portal \
+        polkit \
+        pipewire \
+        pipewire-pulseaudio \
+        wireplumber
+}