fix(udpscope): keep the clock-offset recalibration threshold symmetric

The jitter test fed a receive timestamp that went backwards (1001.02 then
1000.97), putting the second reading 1.03 s from the prediction — twice the
threshold, so not jitter under any reading. That is a digit slip for 1001.97.

It had been worked around by making the threshold one-sided, which passes the
test but never fires when the producer's clock steps forward: the prediction
stays ahead of the wall clock, the error stays negative, and the trace sits in
the future for the rest of the run. Restored std::fabs, corrected the test data,
and added the forward-jump case that the one-sided version silently failed.

Plan amended so the bad data does not come back.

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

Client/udpscope/tests/TimeBaseTest.cpp

@@ -20,8 +20,24 @@ TEST(ClockOffset, HoldsTheOffsetThroughSmallArrivalJitter) {
     ClockOffset off;
     off.map(10.0, 1000.0);                      // offset = 990
 
-    EXPECT_DOUBLE_EQ(off.map(11.0, 1001.02), 1001.0);
-    EXPECT_DOUBLE_EQ(off.map(12.0, 1000.97), 1002.0);
+    // Arrival wanders either side of the prediction. wallSec is a local receive
+    // timestamp, so it only ever advances — jitter shows up as the gap growing
+    // and shrinking, never as the clock going backwards.
+    EXPECT_DOUBLE_EQ(off.map(11.0, 1001.02), 1001.0);   // +0.02 late
+    EXPECT_DOUBLE_EQ(off.map(12.0, 1001.97), 1002.0);   // -0.03 early
+}
+
+// The threshold has to be symmetric. A producer whose clock steps FORWARD (an
+// NTP correction on the producer's host, say) puts the prediction permanently
+// ahead of the wall clock — a one-sided "recalibrate only when wall is ahead"
+// test never fires for it, and the trace sits in the future for the rest of the
+// run.
+TEST(ClockOffset, RecalibratesWhenTheProducerClockJumpsForward) {
+    ClockOffset off;
+    off.map(10.0, 1000.0);                      // offset = 990
+
+    // Producer leaps 100 s ahead while only 1 s of wall time passes.
+    EXPECT_DOUBLE_EQ(off.map(111.0, 1001.0), 1001.0);
 }
 
 TEST(ClockOffset, RecalibratesWhenDriftExceedsTheThreshold) {