feat(events): causal timeline over recorded links only

A bounded walk (six hops, two hundred events) over the links events
actually record: causation ids, parent ids and cited-event evidence,
backwards towards causes and forwards towards consequences, with the edge
kind on every arrow. Nodes carry their cycle for grouping, labelled as
correlation rather than causation, and the artefacts (beliefs, memories,
model calls) they rest on. Cyclic references terminate. Exposed at
GET /api/v1/events/{id}/chain and /events/{id}/timeline, linked from the
event page, and documented.

web/templates/pages/event.html

@@ -8,7 +8,7 @@
 <section class="grid cols-3-1">
   <div class="stack">
     <div class="card">
-      <h2>Causal chain</h2>
+      <h2>Causal chain <a class="small" href="/events/{{$e.ID}}/timeline">full timeline →</a></h2>
       <div class="chain">
         {{if $c.Cause}}<div class="chain-node"><span class="muted small">caused by</span><br><span class="etype cat-{{$c.Cause.Category}}">{{$c.Cause.Type}}</span> <a href="/events/{{$c.Cause.ID}}">{{$c.Cause.Summary}}</a></div><div class="chain-arrow">↓</div>{{end}}
         <div class="chain-node current"><span class="etype cat-{{$e.Category}}">{{$e.Type}}</span> {{$e.Summary}}</div>