Taistamp: Signed TAI64N Timestamps over HTTP

This memo defines Taistamp, an HTTP-accessible service that returns the current time as a TAI64N label, optionally signed with Ed25519. The service is reached at the well-known URI /.well-known/taistamp and is intended for clients that need an authenticated wall-clock reading without running an NTP stack or relying on the client's own clock for TLS certificate validation. The signing scheme uses a length-prefixed, domain-separated framing and publishes verification keys as DNS TXT records in a DKIM-inspired selector layout.