Timers: heuristically bind LTC when the stage layout omits the field

Verified against ProPresenter 21.4: the legacy stage-display protocol
(ptl 610 — the only version PP21 accepts; newer values get 'out of
date') omits stage objects it predates, so a timecode field labeled LTC
never appears in any layout's frame list, and no timecode messages were
observed while idle. Fallback: an unknown uid streaming timecode-shaped
text (a fourth :FF group; not any configured timer's uuid) is bound as
LTC after two advancing samples, and that binding survives layout
refreshes. pp-timers-mock gains --ltc-unlisted to exercise the path.

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

modules/propresenter-timers/server.js

@@ -308,6 +308,9 @@ module.exports = {
         port,
         password: String(config.stagePassword || ''),
         log,
+        // The HTTP poller knows every configured timer's uuid — anything
+        // else streaming timecode-shaped text can be heuristically bound.
+        isTimerUid: (uid) => timerConfigs.has(uid),
         onUpdate(update) {
           stageLtc = update;
           broadcast();