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

static/dashboard.html

@@ -400,7 +400,9 @@ const IND_SHORT = {ema_trend:"EMA", rsi:"RSI", macd:"MACD", bollinger:"BB",
 const ARROW = {STRONG_BUY:"⇈", BUY:"↑", NEUTRAL:"→", SELL:"↓", STRONG_SELL:"⇊"};
 const TF_LABEL = {"5min":"M5", "15min":"M15", "1h":"H1", "4h":"H4"};
 const TF_ORDER = ["5min","15min","1h","4h"];
-const POLL_MS = 20000;
+/* 5 s polling is cheap because fetchJSON uses conditional GETs — Apache
+   answers unchanged files with 304, so most polls transfer no body. */
+const POLL_MS = 5000;
 
 let snapshot = null, news = null;
 let clockOffset = 0;           // serverEpoch - localEpoch
@@ -699,7 +701,10 @@ function renderWire() {
 
 /* ── polling ──────────────────────────────────────────────────── */
 async function fetchJSON(name) {
-  const res = await fetch(`${name}?t=${Date.now()}`, {cache: "no-store"});
+  /* no-cache = revalidate with the server every time (If-Modified-Since);
+     unchanged files come back 304 with no body, so aggressive polling
+     stays nearly free. The old ?t= cache-buster forced full downloads. */
+  const res = await fetch(name, {cache: "no-cache"});
   if (!res.ok) throw new Error(`${name}: HTTP ${res.status}`);
   /* The Date header is stamped at RESPONSE time — snapshot.server_time_epoch
      is stamped at publish time, minutes stale between candle closes. */
@@ -724,13 +729,15 @@ async function poll() {
     tick();  // countdowns appear immediately, not after the first 1 s tick
 
     /* one-shot refetch shortly after the next expected publish, so a
-       fresh scan appears within seconds of the candle close */
+       fast scan appears ~2 s after the publisher wakes instead of
+       waiting out the poll interval (the regular POLL_MS loop covers
+       slower scans) */
     const nextScan = Math.min(...TF_ORDER.map(tf =>
       snapshot.timeframes[tf]?.next_scan_epoch || Infinity));
     if (isFinite(nextScan)) {
       clearTimeout(refetchTimer);
-      const delay = (nextScan - nowServer() + 6) * 1000;
-      if (delay > 0 && delay < POLL_MS) refetchTimer = setTimeout(poll, delay);
+      const delay = (nextScan - nowServer() + 2) * 1000;
+      if (delay > 0) refetchTimer = setTimeout(poll, delay);
     }
   } catch (err) {
     const last = lastPollOk ? ` Last update ${lastPollOk.toLocaleTimeString()}.` : "";