feat(conformance): implement ReadOnly brand and reader generator

- Add ReadOnly<F> phantom brand to core-shared/conformance (compile-time
  enforcement that readers only wrap non-mutating use cases)
- Add isReadOnly runtime predicate for boot-time assertReaderPurity
- Scaffold pnpm turbo gen reader: creates integrations/readers/ with
  interface, implementation, test, barrel, and adds ./reader export
  subpath to package.json

turbo/generators/templates/reader/reader.test.ts.hbs

@@ -0,0 +1,10 @@
+// packages/{{kebabCase feature}}/src/integrations/readers/{{kebabCase feature}}.reader.test.ts
+import { describe, it, expect } from "vitest";
+import { {{pascalCase feature}}Reader } from "@/integrations/readers/{{kebabCase feature}}.reader";
+
+describe("{{pascalCase feature}}Reader", () => {
+  it("can be constructed", () => {
+    const reader = new {{pascalCase feature}}Reader();
+    expect(reader).toBeDefined();
+  });
+});