dashboard: cut page sync latency to ~scan-time + ≤5 s

Three display-side fixes, no signal-logic changes:
- run_cycle publishes snapshot.json BEFORE the news refresh — news I/O
  (FF + 4 RSS + possible DeepSeek calls) took 20-30 s on TTL-expired
  cycles and kept fresh signals off the page (publish_all split into
  publish_snapshot/publish_news, kept for boot)
- page polls every 5 s (was 20 s) using conditional GETs: dropped the
  ?t= cache-buster + no-store so Apache answers unchanged files with
  bodyless 304s
- boundary refetch sniper re-armed: fires 2 s after next_scan_epoch
  regardless of distance (the old delay<POLL_MS guard would have been
  dead code at 5 s polling)

New Module 20 test locks the snapshot-before-news ordering. 352 tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017mbXs9HRawZhD1uZ3ze7zQ

dashboard_publisher.py

@@ -371,18 +371,29 @@ class DashboardPublisher:
         except Exception:
             logger.exception("news refresh failed — keeping last payload")
 
-    def publish_all(self) -> None:
-        now = datetime.now(timezone.utc)
-        publish(build_snapshot_payload(self.snapshots, now),
-                os.path.join(self.out_dir, SNAPSHOT_FILE))
+    def publish_snapshot(self) -> None:
+        publish(build_snapshot_payload(
+            self.snapshots, datetime.now(timezone.utc)),
+            os.path.join(self.out_dir, SNAPSHOT_FILE))
+
+    def publish_news(self) -> None:
         if self.news_payload is not None:
             publish(self.news_payload, os.path.join(self.out_dir, NEWS_FILE))
 
+    def publish_all(self) -> None:
+        self.publish_snapshot()
+        self.publish_news()
+
     def run_cycle(self, tfs: List[Timeframe]) -> None:
         for tf in tfs:  # sequential — see module docstring (cache dedup)
             self.scan_tf(tf)
+        # Snapshot lands BEFORE the news refresh: when the news TTL has
+        # expired, refresh_news does FF + 4 RSS fetches (and possibly
+        # DeepSeek calls) — 20-30 s that must not keep fresh signals off
+        # the page.
+        self.publish_snapshot()
         self.refresh_news()
-        self.publish_all()
+        self.publish_news()
 
     def run_forever(self) -> None:
         self.run_cycle(DASHBOARD_TFS)  # populate the page at boot