The evidence log's head as a C2SP checkpoint

A signed note (c2sp.org/tlog-checkpoint): origin api.riskrouter.eu/tlog/evidence-v2, tree size, base64 root, signed with the log's Ed25519 key. The same tree as GET /api/v2/evidence/head. The tree's hash tiles are served beside it at /tlog/evidence-v2/tile/<L>/<N>[.p/<W>] (c2sp.org/tlog-tiles); entry bundles are not published.