M7: WASM plugins (#101)

Sandboxed WebAssembly plugins whose API is the command registry:

- figcraft-plugin (new crate, L2): the figcraft-plugin.json manifest
  (menu commands with parameters, read/write access, command allowlist)
  and the sandbox: wasmi (pure Rust, no JIT, also in the web build) with
  fuel metering, a memory cap, no WASI and seven host functions (call,
  selection, reply_len, reply_read, log, notify, set_output).
- Engine: plugin.list / plugin.import / plugin.remove / plugin.run,
  plugins.runLast (Alt+Cmd+P; Ctrl+Alt+Shift+P on Windows/Linux) and
  plugins.manage. A run is one undo step (Session::grouped, which
  command.batch now uses too) and is rolled back when it fails. Files,
  other documents, the undo history, dialogs and plugins are off limits;
  read-only plugins can't change the document. Installed plugins live in
  the preferences; development plugins re-read their module every run.
- UI: installed plugins in the Plugins menu, a parameters form drawn
  from the manifest, Manage plugins, Plugins > Development > Import
  plugin from manifest.
- plugins/: a no_std Rust SDK and two example plugins (Tidy names,
  Swatch grid), built for wasm32 by `cargo xtask plugins` (part of
  `cargo xtask wasm`).
- Docs: docs/plugins.md (manifest, permissions, limits, ABI, SDK),
  docs/mcp.md, docs/control-protocol.md, README, ROADMAP, parity.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -93,6 +93,7 @@ fn registry() -> &'static Registry {
         list.extend(crate::component_codegen::specs());
         list.extend(crate::annotations::specs());
         list.extend(crate::distance::specs());
+        list.extend(crate::plugins::specs());
         let mut index = HashMap::new();
         for (i, s) in list.iter().enumerate() {
             if index.insert(s.id, i).is_some() {