🔧 Replace ESLint and Prettier with oxlint and oxfmt

- Drop .eslintrc.cjs and .prettierrc, add .oxlintrc.json and .oxfmtrc.json
- oxlint covers correctness/suspicious/perf/style/restriction categories
  plus import and typescript plugins; oxfmt reads the same .prettierrc-style
  options we had before
- Update lefthook.yml pre-commit jobs to run oxlint/oxfmt instead of
  eslint/prettier
- Update .vscode/extensions.json (oxc replaces eslint/prettier/vitest) and
  settings.json (oxc becomes the default formatter for TS/JS)
- Add 'oxlint', 'oxfmt', 'oxc', 'nodenext' to cspell dictionary
- Rewrite README 'Development' section to describe the new toolchain

.oxfmtrc.json

@@ -0,0 +1,9 @@
+{
+    "semi": true,
+    "singleQuote": false,
+    "trailingComma": "all",
+    "printWidth": 80,
+    "tabWidth": 4,
+    "endOfLine": "lf",
+    "arrowParens": "always"
+}