feat(editor): add spell & grammar check engines with decorations

Wire real spell-checking into the editor via hunspell-wasm (bundled en_US
dictionary) and an optional LanguageTool grammar client.

- spell/: HunspellSpellEngine wrapper, pure spell logic, base64 en_US dict
- grammar/: LanguageToolEngine grammar client and pure grammar logic
- decorations/: collectBlock text assembly, offset-mapped CheckService,
  buildCheckDecorations (flat DecorationSet, inline spell/grammar marks)
- export engines and types from editor index; add hunspell-wasm dep

Verified: typecheck (6 pkgs), tests (all pass), lint (6 pkgs), and web
build all green.

packages/editor/package.json

@@ -21,10 +21,11 @@
   "dependencies": {
     "@pubscript/core": "workspace:*",
     "@tiptap/core": "3.32.0",
+    "@tiptap/extension-text-style": "3.32.0",
     "@tiptap/pm": "3.32.0",
     "@tiptap/react": "3.32.0",
     "@tiptap/starter-kit": "3.32.0",
-    "@tiptap/extension-text-style": "3.32.0"
+    "hunspell-wasm": "^0.3.0"
   },
   "devDependencies": {
     "@types/node": "^22.10.2",