feat: add moonarch-waybar config merger wrapper

Waybar's include directive cannot merge arrays, making per-machine
module customization impossible without duplicating the entire config.

moonarch-waybar merges an optional ~/.config/waybar/userconfig with
the system config, supporting prepend/append on module arrays and
object merge for module definitions. Generates user style.css with
@import of system styles on first run.

System waybar config converted from JSONC to valid JSON for jq
compatibility. Niri startup and hotkey updated to use the wrapper.

defaults/bin/moonarch-waybar

@@ -0,0 +1,59 @@
+#!/bin/bash
+# ABOUTME: Wrapper that merges system waybar config with per-machine userconfig.
+# ABOUTME: Handles array prepend/append that waybar's native include cannot do.
+
+SYSTEM_CONFIG="/etc/xdg/waybar/config"
+SYSTEM_STYLE="/etc/xdg/waybar/style.css"
+USER_DIR="${XDG_CONFIG_HOME:-$HOME/.config}/waybar"
+USERCONFIG="$USER_DIR/userconfig"
+OUTPUT="$USER_DIR/config"
+USER_STYLE="$USER_DIR/style.css"
+
+merge_config() {
+    mkdir -p "$USER_DIR"
+
+    if ! jq -s '
+        .[0] as $sys | .[1] as $user |
+        (($user.prepend // {}) | to_entries) as $prepends |
+        (($user.append  // {}) | to_entries) as $appends |
+        $sys |
+        reduce $prepends[] as $p (.;
+            .[$p.key] = ($p.value + (.[$p.key] // []))
+        ) |
+        reduce $appends[] as $a (.;
+            .[$a.key] = ((.[$a.key] // []) + $a.value)
+        ) |
+        ($user | del(.prepend) | del(.append)) as $extras |
+        . * $extras
+    ' "$SYSTEM_CONFIG" "$USERCONFIG" > "${OUTPUT}.tmp" 2>&1; then
+        local err
+        err=$(cat "${OUTPUT}.tmp")
+        rm -f "${OUTPUT}.tmp"
+        logger -t moonarch-waybar "Config merge failed: $err"
+        notify-send -u critical "moonarch-waybar" "Config merge failed — using system config.\n$err"
+        return 1
+    fi
+
+    mv "${OUTPUT}.tmp" "$OUTPUT"
+}
+
+bootstrap_style() {
+    if [[ ! -f "$USER_STYLE" ]]; then
+        mkdir -p "$USER_DIR"
+        cat > "$USER_STYLE" << 'CSS'
+/* Generated by moonarch-waybar — add custom styles below */
+@import url("/etc/xdg/waybar/style.css");
+CSS
+    fi
+}
+
+if [[ -f "$USERCONFIG" ]]; then
+    if [[ ! -f "$OUTPUT" ]] ||
+       [[ "$USERCONFIG" -nt "$OUTPUT" ]] ||
+       [[ "$SYSTEM_CONFIG" -nt "$OUTPUT" ]]; then
+        merge_config
+    fi
+    bootstrap_style
+fi
+
+exec waybar "$@"