Add stale-while-revalidate completion notification

[marcstraube]

Context

CacheManager returns stale data immediately during revalidation but does not notify the consumer when background revalidation completes with fresh data.

Goal

Notification mechanism so reactive UIs can update when fresh data arrives.

Implementation

1. Add optional onRevalidate callback to cache config
2. Call with fresh data when background fetch completes
3. Alternative: return { data, revalidating: Promise } tuple

Files

• src/cache/CacheManager.ts
• tests/cache/CacheManager.test.ts

Source: AUDIT4 (-1 Defense-in-Depth)