Add transform script for existing Arch+Wayland systems

Extract shared helpers into lib.sh (log, err, confirm, path constants)
and refactor post-install.sh + update.sh to source it.

New transform.sh converts an existing Arch+Wayland system to Moonarch:
pre-flight summary, config backup, DM conflict resolution, PulseAudio
removal, full package install, and hard overwrite of all configs.

Also migrate kanshi from niri spawn-at-startup to systemd user service.

scripts/lib.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+# ABOUTME: Shared helper functions and constants for Moonarch scripts.
+# ABOUTME: Sourced by post-install.sh, update.sh and transform.sh.
+
+# Path constants — BASH_SOURCE[1] resolves to the calling script, not lib.sh itself.
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[1]}")" && pwd)"
+PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
+OFFICIAL_PACKAGES="$PROJECT_DIR/packages/official.txt"
+AUR_PACKAGES="$PROJECT_DIR/packages/aur.txt"
+DEFAULTS_DIR="$PROJECT_DIR/defaults"
+
+# --- Helper functions ---
+
+log() {
+    echo -e "\e[1;34m[Moonarch]\e[0m $*"
+}
+
+err() {
+    echo -e "\e[1;31m[Moonarch ERROR]\e[0m $*" >&2
+}
+
+read_packages() {
+    grep -v '^\s*#' "$1" | grep -v '^\s*$'
+}
+
+confirm() {
+    read -r -p "$1 [y/N] " response
+    [[ "$response" =~ ^[yY]$ ]]
+}
+
+# --- Prerequisite checks ---
+
+check_not_root() {
+    if [[ $EUID -eq 0 ]]; then
+        err "Do NOT run as root. The script uses sudo where needed."
+        exit 1
+    fi
+}
+
+check_pacman() {
+    if ! command -v pacman &>/dev/null; then
+        err "pacman not found — is this really Arch Linux?"
+        exit 1
+    fi
+}