Phase 2 harness wiring: differ --right-app web, e2e target app, dev-controls, cookbook

- differ can serve apps/web on the right side (--right-app web); impersonation no longer follows app-specific redirects
- e2e: E2E_TARGET_APP=web spawns apps/web servers per worker against the shared dbs; leaderboards spec + page object added; expectRouterIdle exported
- apps/web dev-controls endpoints: /auth/impersonate(+stop), /theme (404-redirect GET parity), /refresh-caches, /end-season, /run-routine, /sidenav
- HydrationTestIndicator implements the hydrated/router-idle contract (counts in-flight remote fetches)
- codemods: route-map (routes.ts -> SvelteKit route scaffolds, manifest-aware, idempotent) and remote-scaffold (loader->query, _action branches->commands)
- MIGRATION.md cookbook seeded with the golden read-path pattern and slice learnings; manifest: leaderboards migrated

apps/web-react/e2e/helpers/playwright.ts

@@ -315,7 +315,7 @@ export async function waitForPOSTResponse(page: Page, cb: () => Promise<void>) {
 }
 
 /** Waits until no navigation, revalidation or fetcher is in flight. */
-async function expectRouterIdle(page: Page) {
+export async function expectRouterIdle(page: Page) {
 	// A submit's redirect plus the target page's loaders can exceed the default
 	// expect timeout when the full suite is loading all workers.
 	try {