Question nav: offer followups by reach, not by carrying any chain

`list_qualifying_questions` reduced the visitor's `?chain=` to a
bool, so one followup path unlocked every followup on the site -
redoal's `/watching` (an email-link page whose header says "never in
the nav") showed up for anyone who had just subscribed.

The nav now resolves the chain hash to the one question it verifiably
answered (the same `lookup_node` a `requires_chain` gate uses) and
offers a followup only when it is on that path or further down from
the current page over `action` edges. `content::outcomes_reachable_from`
walks those edges through followup pages only, so a "back to the
start" action ends the walk at the front door instead of fanning out.
Templated actions (`/shape/{curve.key}`) resolve to their dynamic
page's pattern. No schema change: the `followup` flag keeps meaning
"not a front door", and the edges were already in the content.

Also: `main.loading`/`main.not-found` gets the same `svh` ladder as
`.hero-piece`, the last bare `vh` in the stylesheet, so wrapper
browsers that resize the webview as their toolbar animates don't
reflow the placeholder.

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

style/main.css

@@ -147,6 +147,10 @@ a {
 main.loading,
 main.not-found {
   min-height: 60vh;
+  /* svh so wrapper browsers that resize the webview as their toolbar
+     animates (Firefox/Chrome iOS) don't reflow this placeholder - the
+     small viewport is a fixed value. Same ladder as .hero-piece. */
+  min-height: 60svh;
   display: flex;
   flex-direction: column;
   align-items: center;