ci: scoped 100% coverage gates + policy doc

frontend/vitest.config.ts

@@ -9,8 +9,16 @@ export default defineConfig({
       provider: "v8",
       reporter: ["text", "lcov"],
       reportsDirectory: "./coverage",
-      include: ["src/**/*.{ts,tsx}"],
+      // Policy (scoped gate): require 100% coverage on *explicitly listed* unit-testable modules first.
+      // We'll expand this include list as we add more unit/component tests.
+      include: ["src/lib/backoff.ts"],
       exclude: ["**/*.d.ts", "src/**/__generated__/**", "src/**/generated/**"],
+      thresholds: {
+        lines: 100,
+        statements: 100,
+        functions: 100,
+        branches: 100,
+      },
     },
   },
 });