chore(lint): migrate ESLint 8 to 9 with flat config

Replace legacy .eslintrc.json with eslint.config.js (flat config).
Upgrade eslint ^8.56 -> ^9.27, @typescript-eslint/* ^6.16 -> ^8.32,
add @eslint/js. Disable no-undef (TypeScript handles this), prefix
unused catch vars with underscore, replace ternary statement with
if/else in metrics.ts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vveerrgg

.eslintrc.json

@@ -0,0 +1,38 @@
+import eslint from '@eslint/js';
+import tseslint from '@typescript-eslint/eslint-plugin';
+import tsparser from '@typescript-eslint/parser';
+
+export default [
+  {
+    ignores: ['dist/**', 'node_modules/**', 'coverage/**', 'examples/**'],
+  },
+  {
+    files: ['**/*.ts'],
+    languageOptions: {
+      parser: tsparser,
+      parserOptions: {
+        ecmaVersion: 'latest',
+        sourceType: 'module',
+      },
+      globals: {
+        console: 'readonly',
+        process: 'readonly',
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tseslint,
+    },
+    rules: {
+      ...eslint.configs.recommended.rules,
+      ...tseslint.configs.recommended.rules,
+      'no-undef': 'off',
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/explicit-function-return-type': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      }],
+    },
+  },
+];