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

src/chain.rs

@@ -20,6 +20,11 @@ pub const CHAIN_BUCKET: &str = "portal_chains";
 #[derive(serde::Serialize, serde::Deserialize)]
 pub struct ChainNode {
     pub question_id: String,
+    /// Which alternative was answered - what decides where the visitor
+    /// was sent next, and so which followup the chain has earned. Nodes
+    /// indexed before this field existed carry `None`.
+    #[serde(default)]
+    pub alternative: Option<String>,
     pub timestamp_ms: i64,
 }
 
@@ -30,6 +35,7 @@ pub async fn record_node(
     js: &async_nats::jetstream::Context,
     chain_hash: &str,
     question_id: &str,
+    alternative: &str,
     timestamp_ms: i64,
 ) -> anyhow::Result<()> {
     let store = match js.get_key_value(CHAIN_BUCKET).await {
@@ -44,6 +50,7 @@ pub async fn record_node(
     };
     let node = ChainNode {
         question_id: question_id.to_string(),
+        alternative: Some(alternative.to_string()),
         timestamp_ms,
     };
     store.put(chain_hash, serde_json::to_vec(&node)?.into()).await?;