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

packages/core-shared/src/conformance/brand-runtime.ts

@@ -13,7 +13,7 @@
  * commitment, not a mutable flag.
  */
 
-import type { Analyzed, ConsentChecked, RateLimited } from "./brands";
+import type { Analyzed, ConsentChecked, RateLimited, ReadOnly } from "./brands";
 
 type Brand =
   | "__instrumented"
@@ -21,7 +21,8 @@ type Brand =
   | "__audited"
   | "__analyzed"
   | "__consentChecked"
-  | "__rateLimited";
+  | "__rateLimited"
+  | "__readonly";
 
 /**
  * Attaches the brand as a non-enumerable property on the given function.
@@ -75,3 +76,7 @@ export function isRateLimited<F extends object>(
 ): fn is RateLimited<F> {
   return hasBrand(fn, "__rateLimited");
 }
+
+export function isReadOnly<F extends object>(fn: unknown): fn is ReadOnly<F> {
+  return hasBrand(fn, "__readonly");
+}

packages/core-shared/src/conformance/brands.ts

@@ -12,3 +12,9 @@ export type Captured<F> = F & { readonly __captured: true };
 export type Analyzed<F> = F & { readonly __analyzed: true };
 export type ConsentChecked<F> = F & { readonly __consentChecked: true };
 export type RateLimited<F> = F & { readonly __rateLimited: true };
+/**
+ * Brand for use cases declared `mutates: false`. Readers only accept
+ * `ReadOnly`-branded use cases in their constructor — prevents mutating
+ * use cases from being wired into a reader at compile time.
+ */
+export type ReadOnly<F> = F & { readonly __readonly: true };

packages/core-shared/src/conformance/index.ts

@@ -4,6 +4,7 @@ export type {
   Analyzed,
   ConsentChecked,
   RateLimited,
+  ReadOnly,
 } from "./brands";
 export type { FeatureManifest, UseCaseManifest } from "./define-feature";
 export { defineFeature } from "./define-feature";
@@ -31,6 +32,7 @@ export {
   isAnalyzed,
   isConsentChecked,
   isRateLimited,
+  isReadOnly,
 } from "./brand-runtime";
 export { ConformanceError } from "./conformance-error";
 export { assertFeatureConformance } from "./assert-bindings";