D1 Session Read Replica

Cloudflare D1 read replication done so the write-then-redirect-then-read flow stays correct. Sequential consistency is scoped to one D1DatabaseSession, which dies with the Worker invocation, so a POST that INSERTs and answers 303 hands the follow-up GET a fresh unconstrained session that can read a replica behind the write. This carries session.getBookmark() across the redirect in a Set-Cookie, since a browser drops the redirect's custom headers and replays only cookies. Includes the constraint policy per route type (first-primary on writes to protect the read half of a read-modify-write, the incoming bookmark on the owner's dashboard, an explicit anonymous opt-out on the public cached feed), a commit that no-ops on a null bookmark and still fires on the 429 path, rejection of client-supplied constraint literals in the bookmark slot, and D1Meta served_by_primary logging.