Timers: real-time frame-accurate LTC display

The LTC card repainted only on server anchors (~4x/s), so the frame
counter jumped in ~7-frame steps. Now the client counts every frame
locally between anchors and re-syncs on each one — frame-accurate and
smooth, with no extra network traffic.

- Server anchors carry ageMs (how old the decoded timecode is) so clients
  can pin the anchor to their own clock. ltc-listener reports it while
  running; composeLtc passes it through (listener) or derives it (reader).
- Client does exact SMPTE frame<->timecode math (parseTc/tcToFrames/
  framesToTc, incl. 29.97 drop-frame minute counting) and advances the
  display in a requestAnimationFrame loop at the LTC's own rate.
- Robust when rAF is throttled (backgrounded tab): render() paints the
  extrapolated frame at the 4x/s anchor rate too, so the display never
  freezes — the rAF loop only refines it to every frame when visible.
- Anchor jitter can't tick the counter backwards (small regressions
  clamped; genuine seeks/re-strikes pass); freshness cap stops freewheel
  into phantom frames when anchors stop arriving.

Verified: SMPTE math against the encoder over 200k drop-frame frames plus
round-trips; the extrapolation pipeline simulated at 30/25/29.97DF showing
strict +1-per-frame advance and correct DF boundary crossing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/propresenter-timers/server.js

@@ -132,7 +132,7 @@ module.exports = {
       if (!ltcEnabled) {
         return { supported: false, source: '', time: '', receiving: false, status: '', fps: 0, df: false, note: 'LTC listener disabled' };
       }
-      const audioVerdict = (src, tag) => ({
+      const audioVerdict = (src, tag, ageMs) => ({
         supported: true,
         source: tag,
         time: src.time,
@@ -140,16 +140,19 @@ module.exports = {
         status: src.state,
         fps: src.fps,
         df: src.df,
+        // While running: how old this timecode is, so clients can pin the
+        // anchor to their own clock and count frames between updates.
+        ...(src.state === 'running' && Number.isFinite(ageMs) ? { ageMs: Math.max(0, ageMs) } : {}),
         note: '',
       });
       // The built-in listener while it has actual data …
       if (listenerLtc && !listenerLtc.error && listenerLtc.state !== 'nosignal') {
-        return audioVerdict(listenerLtc, 'listener');
+        return audioVerdict(listenerLtc, 'listener', listenerLtc.ageMs);
       }
       // … else a fresh remote reader (during migrations both may run —
       // real data beats no data) …
       if (ingest && Date.now() - ingest.at < INGEST_STALE_MS) {
-        return audioVerdict(ingest, 'reader');
+        return audioVerdict(ingest, 'reader', Date.now() - ingest.at);
       }
       // … else a healthy-but-silent listener is believed over the indirect
       // sources: no audio means the LTC really stopped.