🔧 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

.oxlintrc.json

@@ -0,0 +1,33 @@
+{
+    "$schema": "./node_modules/oxlint/configuration_schema.json",
+    "plugins": [
+        "typescript",
+        "unicorn",
+        "oxc",
+        "import"
+    ],
+    "categories": {
+        "correctness": "error",
+        "suspicious": "error",
+        "perf": "warn",
+        "style": "warn",
+        "restriction": "error",
+        "nursery": "off"
+    },
+    "rules": {},
+    "env": {
+        "builtin": true,
+        "es2024": true,
+        "node": true
+    },
+    "overrides": [
+        {
+            "files": ["**/*.test.ts"],
+            "rules": {
+                "no-unused-expressions": "off",
+                "no-empty-file": "off"
+            }
+        }
+    ],
+    "ignorePatterns": ["dist", "node_modules", "coverage"]
+}