Announced pages: event windows, header announcements, summary tasks

A question may carry event: {starts, duration, place}. While the
window is open the page is announced in a strip at the top of every
header (name, when, 'in 3 days'), soonest first, and kept out of the
footer nav; when it closes the page becomes a followup - only a
visitor carrying an answer chain still sees it.

announce.rs keeps one record per event page in the runtime-owned
portal_events bucket (built-in state graph: announced ->
awaiting_summary -> summarized, content may override) and, on a
one-minute idempotent sweep, moves ended windows to awaiting_summary,
publishing the transition on portal.answers.submitted as 'Summary
due' - the post-what-happened task a review desk picks up. Lint
warns when event pages exist but nothing reads portal_events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

style/main.css

@@ -206,6 +206,56 @@ main.not-found {
   min-height: 55svh;
 }
 
+/* announcements - live event pages, a strip above the hero copy.
+   Header, not footer: the one place a page claims attention before
+   the question is asked. */
+.announce {
+  width: 100%;
+  display: flex;
+  flex-wrap: wrap;
+  justify-content: center;
+  gap: 0.6rem 1.2rem;
+  margin: -2rem 0 0.5rem;
+  font-size: 0.88rem;
+  position: relative;
+  z-index: 1;
+}
+
+.announce-item {
+  display: inline-flex;
+  flex-wrap: wrap;
+  align-items: baseline;
+  gap: 0.35rem 0.7rem;
+  padding: 0.45rem 0.9rem;
+  border: 0.06rem solid var(--line);
+  border-left: 0.2rem solid var(--accent);
+  border-radius: 0.3rem;
+  color: var(--ink);
+  text-decoration: none;
+  background: var(--paper);
+  transition: border-color 120ms;
+}
+
+.announce-item:hover,
+.announce-item[aria-current="page"] {
+  border-color: var(--accent);
+}
+
+.announce-name {
+  font-weight: 600;
+}
+
+.announce-when {
+  color: var(--ink-dim);
+}
+
+.announce-relative {
+  color: var(--accent);
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  font-size: 0.72rem;
+}
+
 /* alternatives */
 
 /* position + z-index on these three: everything that scrolls over the