Two correctness/defense fixes plus regression tests in the existing
in-memory drizzle test file.
1. Reverse-direction idempotency. The sender-side path in social.ts
checks BOTH directions of friend_requests and returns 409
incoming_request_exists when an opposite-direction row exists. The
receiver only matched from->to, so cross-fire (alice@A and bob@B both
click "add friend" near-simultaneously) produced two opposite
pending rows on each instance. The receiver now silent-accepts when
either direction matches a pending row, mirroring the sender's
both-direction check.
2. Self-target guard (defense-in-depth). Reject events whose
from-identity equals to-identity (after normalizeOriginForCompare)
with a new receiver-acknowledged 4xx code self_target_invalid.
Sender's local cannot_friend_self should catch this, but the
receiver does not trust upstream validation. Added to
TERMINAL_REJECTION_REASONS so the standard rollback fires
(mapped client-side to peer_rejected). Logged at console.warn.
Spec updates: social.md inbound contract now documents both-direction
idempotency and the self-target guard; federation.md and the
s2s-friend-add design spec list the new terminal rejection reason.
Jannis Braun
b698ded47d