fix(udpscope): reconstruct lost accumulated bursts from the packet counter

Review found the decoder was estimating something the wire states exactly.
FrameView::counter increments once per update, so a gap of g means g-1 lost
datagrams; reinstating their duration restores the hole precisely, with no
threshold and no dependence on arrival time. The arrival-anchor comparison
survives only as a backstop for what the counter cannot express — a producer
restart, a counter stuck at zero, a wrong declared rate — and can no longer
step a signal's timestamps backwards, which the ring and trigger forbid.

Also from review: guard the time-signal lookup against a frame carrying more
signals than the installed table, and give FrameBuilder a counter parameter.
Leaving it at zero had hidden the counter rules from every test, and made the
hrt-gap test vacuous — under uniform arrivals the hrt path and packetBurst
agree by construction, so it could not tell which branch answered. Its
arrivals now carry zero-mean jitter.

Each new assertion was proven non-vacuous by sabotage: dropping the gap term,
the backward guard, or the hrt branch fails exactly its own test.

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

Client/udpscope/FrameDecoder.h

@@ -54,12 +54,11 @@ private:
         bool        lastAccValid    = false;
         uint32_t    prevAccCount    = 0;
         /** For accumulated scalars with a declared sampling rate: end timestamp
-         *  of the most recently emitted burst. The next burst is PREDICTED to
-         *  start one sample period after it — immune to arrival-time jitter —
-         *  but the prediction is discarded when arrival time disagrees with it
-         *  by more than a delivery backlog can explain, so packet loss cannot
-         *  displace the trace permanently. */
-        double      lastEmittedEnd  = 0.0;
+         *  of the most recently emitted burst, and the packet counter it came
+         *  from. The next burst is chained onto that end, with the counter gap
+         *  reinstating the exact duration of any lost datagrams. */
+        double      lastEmittedEnd   = 0.0;
+        uint32_t    lastCounter      = 0u;
         bool        lastEmittedValid = false;
     };