fix(udpscope): bound accumulated-burst chaining so packet loss cannot displace the trace

Forward-chaining each accumulated burst onto the previous one suppresses
arrival jitter, but an unchecked chain never recovers: one lost datagram, or a
declared sampling rate that differs from the producer's real one, dates every
later sample early for the rest of the run. The chain is now a prediction,
compared each packet against the arrival anchor and abandoned beyond
kBurstResyncThresholdS, which bounds the error instead of accumulating it.

Plan amended so the hrt-fit fallback (unusable here: the fit needs 32 packets
and is itself corrupted by bursty arrivals) cannot come back.

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

Client/udpscope/FrameDecoder.h

@@ -54,8 +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, used as a forward-chain anchor
-         *  that is immune to arrival-time jitter. */
+         *  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;
         bool        lastEmittedValid = false;
     };