Extract engine from citation-evidence umbrella (CENG-WP-0001)

Bootstrap @citation-evidence/engine as a standalone TypeScript package
with shared types and engine services copied from the umbrella MVP.
All 89 tests pass with lint and typecheck clean.

eslint.config.js

@@ -0,0 +1,48 @@
+// ESLint flat config — enforces shared/engine dependency boundary.
+
+import js from "@eslint/js";
+import tseslint from "typescript-eslint";
+import boundaries from "eslint-plugin-boundaries";
+import importPlugin from "eslint-plugin-import";
+import globals from "globals";
+
+export default tseslint.config(
+  {
+    ignores: ["dist/", "node_modules/", "coverage/", "**/*.d.ts"],
+  },
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    files: ["src/**/*.{ts,tsx}"],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: "module",
+      globals: { ...globals.node },
+    },
+    plugins: {
+      boundaries,
+      import: importPlugin,
+    },
+    settings: {
+      "import/resolver": {
+        typescript: { project: "./tsconfig.json" },
+      },
+      "boundaries/elements": [
+        { type: "shared", pattern: "src/shared/**" },
+        { type: "engine", pattern: "src/engine/**" },
+      ],
+    },
+    rules: {
+      "boundaries/element-types": [
+        2,
+        {
+          default: "disallow",
+          rules: [
+            { from: "shared", allow: [] },
+            { from: "engine", allow: ["shared"] },
+          ],
+        },
+      ],
+    },
+  },
+);