hs2019 signatures hashed with SHA-512, and SHA-512 digests

A draft-cavage hs2019 signature leaves the hash to the key: with an RSA key it is tried with SHA-256, as nearly everyone
signs, then with SHA-512, as some do; rsa-sha256 stays SHA-256 only. A Digest of SHA-512= is checked as SHA-256= is.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

docs/ROADMAP.md

@@ -692,7 +692,7 @@ it, raw where it doesn't.
     Mitra refuses a proof by a key it has not read and does not read the actor again; a forwarded activity with a
     proof its actor's key verifies is taken without reading it again; Mastodon accepts PrivaPub's, so Activity-Relay's
     forwards reach it);
-  - `hs2019` with SHA-512.
+  - `hs2019` with SHA-512: **done 2026-10-06** (and `SHA-512=` digests).
 - **Discovery:**
   - a relay client for both relay styles: **done 2026-10-05/06** (`Federation:Relays`; forwarded posts read again
     from their origin, announces unwrapped; personas' public posts sent to them, owner decision; checked live against