Friends-only access via wac conditions #31

Use wac conditions (secure-access-conditions) on chat resources so only listed contacts can post. Extends #16 (Read+Append hardening) with identity-based conditions: friends list as vCard:Group, ACL grants acl:Append to the group + optional time-bound conditions.

Server-side: JSS already implements secure-access-conditions. Other Solid servers can implement the same primitive incrementally.

Outcome: spam-free chat by default — only contacts you've explicitly added can post to your chat resource.

Refs:

• ACL should be set to Read Append only #16 (ACL Read/Append hardening)
• To avoid impersonation each message should be signed #17 Feature: Add cryptographic signatures to chat messages #18 (message authenticity — adjacent)
• https://webacl.org/secure-access-conditions/
• JavaScriptSolidServer/JavaScriptSolidServer (server impl)