M13.1: ScriptUI frontend (dialogs, palettes, dockable panels), script menus, example plug-in, docs

- ui-egui: scriptui_view draws script windows at engine layout bounds; PanelKind::ScriptPanel; File ▸ Scripts and Window menu list scripts/panels (egui + native menus)
- examples/plugins/posterize-bands (WebAssembly, API v1); host plug-in tests; desktop loads <config>/Plug-ins
- MCP: script_ui and history tools; docs/plugins.md, agents.md, parity, architecture, ATTRIBUTION

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/commands/file_more.rs

@@ -828,7 +828,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Run Script File...",
             ["File", "Scripts"],
             None,
-            "{path (.jsx/.js JavaScript, or a .jsonl/.json command script) | steps: [{command, params}] | name (an installed or sample script, see file.scripts.list)}",
+            "{path (.jsx/.js JavaScript, or a .jsonl/.json command script) | name (an installed or sample script, see file.scripts.list) | steps: [{command, params}]}",
             always,
             run_script
         ),

crates/engine/src/menus.rs

@@ -757,6 +757,7 @@ Effect
   Remove All | effect.removeAll
   ---
   Manage Effects... | effect.manage
+  Load Effect Plug-in... | effect.plugins.load
   ---
   @effects
 Animation