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

modules/twm/mango.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+set -uo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=../../lib/common.sh
+source "$HERE/../../lib/common.sh"
+# shellcheck source=../../lib/session.sh
+source "$HERE/../../lib/session.sh"
+
+if is_debug; then set +e; else set -e; fi
+
+step "Tiling window manager: MangoWC"
+rule
+
+# MangoWC is not in the Fedora repos; it is packaged in Terra.
+add_terra_repo
+
+info "Installing MangoWC..."
+if ! sudo dnf install -y mangowc; then
+    err "mangowc is not available from Terra on this release."
+    err "Alternatives: copr dennemann/MangoWC, or build from source:"
+    err "    https://github.com/GravityShark/mangowc"
+    exit 1
+fi
+
+install_packages fuzzel alacritty
+install_optional xdg-desktop-portal-wlr
+install_optional xwayland-satellite
+
+install_session_prereqs
+install_nerd_font JetBrainsMono
+
+# MangoWC's session entry is not guaranteed by the package; write one if absent.
+write_wayland_session mangowc "MangoWC" "mangowc"
+
+install_packages sddm
+switch_display_manager sddm
+
+ok "MangoWC setup complete. Log out and pick MangoWC in SDDM."