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/index.ts.hbs

@@ -0,0 +1,3 @@
+// packages/{{kebabCase feature}}/src/integrations/readers/index.ts
+// Public surface: type-only export. Implementation is internal.
+export type { I{{pascalCase feature}}Reader } from "./{{kebabCase feature}}.reader.interface";