Question nav: a followup is offered only once it is earned

The 0.3.34 rule listed every followup "further down" from the current
page, so the front door offered the thank-you pages before anything
was submitted, and a followup one typed in offered the ones beyond
it. The chain half was weaker than it looked too: it matched the
question the chain answered, which is a front door anyway.

Now a followup is offered only when the visitor has earned it: their
verified chain answered the alternative that leads there (the chain
index records the alternative from here on; older nodes fall back to
every recording alternative of the answered question, and nothing
beyond it), or it hangs off an earned page by a link that records
nothing. A page behind an answer they have not given is never offered.
The walk ends at any front door, so "back to the start" never fans out.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L4jrCgLiKKHAEFuZUJjckH

CHANGELOG.md

@@ -6,6 +6,7 @@ content repos pin the tag they run (see the end of this file).
 
 ## Unreleased
 
+- Question nav: a followup is offered only once earned - the verified chain answered the alternative leading there, or a link that records nothing leads on from an earned page. Standing on a page no longer unlocks the pages behind its forms; the chain index now records which alternative was answered
 - One bounded HTTP client for every server-side fetch: 20 s budget, size caps per kind of fetch (`src/http.rs`)
 - Uploads are refused past 20 MB while streaming, and the route body limit that had capped them at axum's 2 MB default is raised to match
 - `tests/jetstream.rs`: the store, the event log and the projection tested against a real JetStream (CI starts a throwaway one)