Redeem a room invite (x402-paid; the verified payer becomes the redeemer)

An x402 paywall resource priced as the room_seat SKU ($0.01) — the ONE resource in this API advertised on the TESTNET network ONLY (every other paid resource advertises both networks): the seat is an onboarding handshake, not revenue. The verified payer of the settled payment becomes the redeemer; no SIGN-IN-WITH-X header is required or accepted. A control-plane session, grant key, or service key is refused 403 ROOM_INVITE_REDEEM_REQUIRES_WALLET before any 402 challenge is issued. A same-payer replay of an already-redeemed invite is answered 200 with deduplicated:true before any payment is challenged — paying twice never charges twice. A refusal AFTER settlement (wrong secret, expired, revoked, or already redeemed by a different principal) refunds the seat. On success the redeemer becomes a permanent viewer of the inviting org, never a vault writer.