feat(core-analytics): add IAnalytics interface, types, and NoopAnalytics

Replaces generator placeholder with IAnalytics interface (track, identify,
pageView, flush), AnalyticsAttributeValue + AnalyticsUser types, and
NoopAnalytics implementation. Adds sibling tests covering all four methods
with 100% coverage. All conformance + coverage gates pass.

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

packages/core-analytics/src/index.ts

@@ -1,2 +1,6 @@
-// placeholder — populated by story 01-scaffold-core-analytics-package
-export {};
+export type {
+  AnalyticsAttributeValue,
+  AnalyticsUser,
+  IAnalytics,
+} from "./analytics.interface";
+export { NoopAnalytics } from "./noop-analytics";