Next.js 15 Async Request Context & Server Actions Invariants

# RFC-001: Next.js 15 Async Request Context & Server Actions Invariants

## 1. Abstract
This specification defines strict architectural constraints for Next.js 15 applications utilizing the App Router architecture. It eliminates common agent hallucinations where models generate synchronous cookie access, invalid router imports, and unvalidated Server Actions.

## 2. Invariant Specifications (RFC-2119)

### 2.1 Asynchronous Request Contexts
In Next.js 15, the following APIs return Promises and MUST be awaited:
```typescript
// MUST: Await cookies and headers
const cookieStore = await cookies();
const token = cookieStore.get("session_token");

const headerStore = await headers();
const authHeader = headerStore.get("authorization");
```

### 2.2 Navigation Isolation
Under no circumstances should 'next/router' be imported inside App Router components:
```typescript
// MUST NOT
import { useRouter } from 'next/router';

// MUST
import { useRouter, usePathname, useSearchParams } from 'next/navigation';
```

## 3. Backward Compatibility
Pages router files placed under `pages/` remain exempt, but mixed directory projects must separate App Router concerns completely.