feat(core-shared): add NoopRateLimit and InMemoryRateLimit implementations

NoopRateLimit always allows with Infinity remaining — zero-overhead default
for apps without a wired rate-limit impl. InMemoryRateLimit uses a Map-backed
fixed-window with check-at-read expiry and an injected clock for testability.
Both exported from the core-shared barrel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-shared/src/index.ts

@@ -3,3 +3,4 @@ export { toIsoString } from "./lib/date";
 export * from "./audit";
 export * from "./di";
 export * from "./instrumentation/index";
+export * from "./rate-limit/index";