feat(core-analytics): scaffold @repo/core-analytics via generator

Add the `pnpm turbo gen core-package analytics` generator template and
run it to scaffold the @repo/core-analytics workspace package. The
package lands in placeholder state (empty barrel export) ready for the
IAnalytics + NoopAnalytics implementation in the next commit.

Includes:
- turbo/generators/templates/core-package/analytics/ templates
- turbo/generators/config.ts analytics generator registration
- packages/core-analytics/ placeholder scaffold
- apps/web-next/next.config.mjs transpilePackages entry

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

packages/core-analytics/src/index.ts

@@ -0,0 +1,2 @@
+// placeholder — populated by story 01-scaffold-core-analytics-package
+export {};