Consolidate per-request ALS scopes for reduced overhead on Workers

[Divkix]

Problem

Every App Router request in vinext runs through 5-6 nested AsyncLocalStorage.run() calls, each from a different shim module:

1. runWithHeadersContext (headers.ts)
2. runWithNavigationContext (navigation-state.ts)
3. _runWithCacheState (cache.ts)
4. runWithPrivateCache (cache-runtime.ts)
5. runWithFetchCache (fetch-cache.ts)
6. runWithExecutionContext (request-context.ts) — conditional, Workers only

Each ALS scope push/pop has measurable cost on Workers isolates. For simple static routes that don't use headers, cookies, or caching, all contexts are created unnecessarily.

Solution

Consolidate into a single UnifiedRequestContext backed by one ALS instance. Each shim module checks isInsideUnifiedScope() first and reads its sub-fields from the unified store, falling back to its own standalone ALS for backward compatibility (SSR environment, Pages Router).

Scope

Phase 1 (this issue)

• App Router only — main request handler + ISR background regeneration
• Dual-path migration: unified scope first, standalone ALS fallback
• No changes to per-call ALS scopes (cacheContextStorage, _unstableCacheAls)

Phase 2 (follow-up)

• Pages Router consolidation (router-state, head-state)
• Production server (prod-server.ts) alignment

Related

• PR: refactor: consolidate 5 nested ALS scopes into unified request context #450