[client] x11: recover stalled present cadence

Reseed the synthetic Present target window when compositor MSC advances
outside the queued range. This prevents a long compositor stall from
permanently exhausting JIT cadence callbacks.

Mark the recovery sample as discontinuous so calibration does not treat the
MSC jump or completion burst as renderer skipping.

client/displayservers/X11/x11.h

@@ -78,6 +78,7 @@ struct X11DSState
   _Atomic(uint64_t) presentMsc, presentUst;
   _Atomic(uint64_t) presentPeriod;
   _Atomic(uint64_t) nominalPeriod;
+  _Atomic(bool)     presentDiscontinuity;
   uint32_t          presentSerial;
   Pixmap            presentPixmap;
   XserverRegion     presentRegion;