Stale state/cache invalidation

Cached/replicated data is outdated due to missing or incorrect invalidation or revalidation.

Define clear TTL/Revalidation rules; emit cache-bust events; use ETag/If-Match; align client and server caching semantics

Observable signs that may indicate Stale state/cache invalidation:

• Users see old data after update until cache expires.
• Inconsistent views across nodes/regions.
• Conditional requests not revalidating as expected.