fix premature FINALIZED gate on G-code mode change

G64/G61 mode changes called tpFlushCompressor_9D which sealed the queue,
causing the optimizer to finalize the tail segment immediately with wrong
boundary conditions (full length, vf=0) before the successor segment and
its blend trimming arrived.

RT would then activate the segment, accelerate to cruise speed, and when
the blend later trimmed the segment to ~half its length, Ruckig had to
emergency-brake in the tiny remaining distance — producing a jerk spike.

Fix: add tpFlushCompressorNoSeal_9D that flushes the compressor without
setting queue_sealed.  Used for mode changes where new motion immediately
follows.  tpFlushCompressor_9D (with seal) is reserved for true sync
points: dwell, M0/M1, program end.

Author: Luca Toniolo

src/emc/motion_planning/motion_planning_9d_userspace.cc

@@ -241,16 +241,29 @@ extern "C" int tpFlushCompressor_9D(TP_STRUCT *tp)
 {
     int result = compressorFlush(tp);
 
-    // Seal the queue: interpreter is at a sync point (tool change, dwell,
-    // mode change, program end).  No more motion segments are coming until
-    // the next tpAddLine_9D/tpAddCircle_9D call clears this flag.
-    // This lets the optimizer finalize the tail segment immediately.
+    // Seal the queue: interpreter is at a true sync point (dwell, tool
+    // change, M0/M1, program end).  No more motion will arrive until the
+    // next tpAddLine_9D/tpAddCircle_9D clears this flag.  Lets the
+    // optimizer finalize the tail segment immediately.
     tp->queue_sealed = 1;
     g_needs_replan = true;
 
     return result;
 }
 
+extern "C" int tpFlushCompressorNoSeal_9D(TP_STRUCT *tp)
+{
+    // Flush compressor (emit any buffered segments) but do NOT seal the
+    // queue.  Used for G-code mode changes (G64, G61, tolerance updates)
+    // where new motion segments are expected immediately after.
+    // Sealing here would cause the optimizer to prematurely finalize the
+    // tail segment with wrong boundary conditions before the successor
+    // segment and its blend trimming arrive.
+    int result = compressorFlush(tp);
+    g_needs_replan = true;
+    return result;
+}
+
 /**
  * @brief Reset compressor state without flushing (discard any buffered segments).
  *

src/emc/task/taskintf.cc

@@ -1629,7 +1629,11 @@ int emcTrajSetTermCond(int cond, double tolerance)
     if (tp) {
         // Flush segment compressor before mode change — buffered segments
         // were accumulated under the old mode/tolerance.
-        tpFlushCompressor_9D(tp);
+        // Use NoSeal variant: new motion segments are expected immediately
+        // after the mode change, so the queue is NOT at a sync boundary.
+        // tpFlushCompressor_9D (with seal) would prematurely finalize the
+        // tail segment before its blend trimming arrives, causing a spike.
+        tpFlushCompressorNoSeal_9D(tp);
         tp->termCond = cond;
         tp->tolerance = tolerance;
     }

src/emc/tp/tp.h

@@ -102,7 +102,8 @@ int tpAddDwell(TP_STRUCT * const tp, double seconds, struct state_tag_t tag);
  * Emits any buffered compressed segment. Call at program end, mode changes,
  * or before non-line segments (circles, dwells).
  */
-int tpFlushCompressor_9D(TP_STRUCT *tp);
+int tpFlushCompressor_9D(TP_STRUCT *tp);      /* flush + seal (sync points only) */
+int tpFlushCompressorNoSeal_9D(TP_STRUCT *tp); /* flush only, no seal (mode changes) */
 void tpResetCompressor_9D(void);
 
 /**