fix: a clock glitch is bridged and a channel is known by name, so a capture keeps its t0

Two defects behind one report ("a trigger during a capture overrides the first,
and the hold-off does not apply"); the second is also what made a spike in one
channel look like the acquisition resetting.

The trigger engine keyed its invalidation on the trigger channel's *index*, which
every rebuild of the channel list renumbers -- enabling a signal that sorts before
it is enough. Any rebuild therefore abandoned an open capture and set no hold-off,
so the next edge inside the post window became t0. It keys on the channel's name
now, which changes only when the source genuinely does.

A packet whose stamps jump more than 256 packet spans ahead is now rejected as a
fault and *bridged* at the declared cadence, instead of poisoning the monotonic
mark (which pinned every later packet flat at the glitched value) and condemning
the next good packet. Only four consecutive failures abandon the declared time
signal, and that fallback no longer wipes the rings: the arrival clock is anchored
to continue from the monotonic mark, so the history is still on one timeline --
the wipe is what made one glitching packet look like the stream restarting from 0.
The declared unit is resolved before those checks, since they weigh the stamps
against a cadence in seconds.

Guards: UDPSCOPE_PROBE_TRIGRENUM_MS prints t0_held=1 through the real widgets,
and the mock's --time-glitch-at/--time-glitch-by injects the clock fault.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/model.cpp

@@ -501,20 +501,6 @@ void ScopeModel::live_window_locked(double lo_s, double hi_s, u64& first, u64& l
   if (last < first) last = first;
 }
 
-void ScopeModel::reset_live_history() {
-  std::lock_guard<std::mutex> lock(mutex_);
-  reset_live_history_locked();
-}
-
-void ScopeModel::reset_live_history_locked() {
-  BufferStore& src = live();
-  src.time().clear();
-  for (std::size_t c = 0; c < channels_.size(); ++c) {
-    src.values(c).clear();
-    src.channel(c).invalidate();
-  }
-}
-
 void ScopeModel::publish_pause_view(double lo_s, double hi_s, double x_offset) {
   // Timed from before the lock: the wait for it is part of what freezing a view costs the thread
   // doing it, and this one is the UI's.