Secret rotation may not reach members pruned from active list

[sanity]

Problem

PR #109 introduced a dual-state membership model where members with valid invitations may not be in the active members list (they get pruned by post_apply_cleanup if they have no recent messages).

When a member is pruned from the active list but still has a valid invitation:

1. They won't receive rotated secrets during a secret rotation
2. When they rejoin (send first message), they might not be able to decrypt messages encrypted with the rotated secret

Steps to Reproduce (Theoretical)

1. User accepts invite to a private room
2. User gets pruned from active members list (no recent messages)
3. Room owner rotates the room secret
4. User sends their first message (re-added to active list)
5. User may not be able to decrypt messages encrypted with the new secret

Expected Behavior

Members with valid invitations should either:

• Not be pruned if they have encrypted secrets, OR
• Receive the rotated secret when they rejoin

Suggested Test

User accepts invite → gets pruned → owner rotates secret → user sends message → verify decryption works

Context

Found during architectural review of PR #109. The build_rejoin_delta logic handles re-adding members, but secret distribution during rotation may not account for pruned-but-authorized members.

[AI-assisted - Claude]