Add eslint-plugin-boundaries with layered dependency rules

eslint.config.js

@@ -3,6 +3,7 @@
 import js from "@eslint/js";
 import tseslint from "typescript-eslint";
 import unusedImports from "eslint-plugin-unused-imports";
+import boundaries from "eslint-plugin-boundaries";
 
 export default [
   {
@@ -52,4 +53,57 @@ export default [
       "no-console": ["warn", { allow: ["warn", "error"] }],
     },
   },
+  {
+    files: ["src/**/*.{ts,tsx}"],
+    plugins: {
+      boundaries,
+    },
+    settings: {
+      "boundaries/elements": [
+        { type: "routes", pattern: "src/routes/*" },
+        { type: "mf", pattern: "src/mf/*" },
+        { type: "features", pattern: "src/features/*", capture: ["feature"] },
+        { type: "ui", pattern: "src/ui/*" },
+        { type: "shared", pattern: "src/shared/*" },
+        { type: "observability", pattern: "src/observability/*" },
+        { type: "i18n", pattern: "src/i18n/*" },
+        { type: "env", pattern: "src/env/*" },
+      ],
+    },
+    rules: {
+      // Design spec §1.2 layered dependency direction:
+      // features/ cannot import routes/ or mf/
+      // ui/ cannot import features/
+      // shared/ cannot import features/, routes/, mf/, observability/
+      // observability/ cannot import features/, routes/, mf/
+      "boundaries/element-types": [
+        "error",
+        {
+          default: "allow",
+          rules: [
+            {
+              from: "features",
+              disallow: ["routes", "mf"],
+              message: "Features must not import from routes/ or mf/. Use the HostContract or a shared module instead.",
+            },
+            {
+              from: "ui",
+              disallow: ["features", "routes", "mf"],
+              message: "UI layer must not import from features/, routes/, or mf/. UI is consumed by features, not the other way around.",
+            },
+            {
+              from: "shared",
+              disallow: ["features", "routes", "mf", "observability"],
+              message: "Shared modules must not import from features/, routes/, mf/, or observability/.",
+            },
+            {
+              from: "observability",
+              disallow: ["features", "routes", "mf"],
+              message: "Observability modules must not import from features/, routes/, or mf/.",
+            },
+          ],
+        },
+      ],
+    },
+  },
 ];