Fix/Improve junction deviation

- Respect axis max acceleration limits instead of forcing a fixed acceleration value.
- The `junction_unit_vec` ensures proper handling of entry and exit speeds even when the axes involved have different limits.

Marlin/Configuration_adv.h

 //#define JUNCTION_DEVIATION
 #if ENABLED(JUNCTION_DEVIATION)
   #define JUNCTION_DEVIATION_MM 0.02  // (mm) Distance from real junction edge
-  #define JUNCTION_ACCELERATION 1000  // (mm/s²) Maximum centripetal acceleration
   //#define JUNCTION_DEVIATION_INCLUDE_E
 #endif
 

Marlin/src/config/default/Configuration_adv.h

 //#define JUNCTION_DEVIATION
 #if ENABLED(JUNCTION_DEVIATION)
   #define JUNCTION_DEVIATION_MM 0.02  // (mm) Distance from real junction edge
-  #define JUNCTION_ACCELERATION 1000  // (mm/s²) Maximum centripetal acceleration
   //#define JUNCTION_DEVIATION_INCLUDE_E
 #endif
 

Marlin/src/inc/SanityCheck.h

 #elif defined(JUNCTION_DEVIATION_FACTOR)
   #error "JUNCTION_DEVIATION_FACTOR is now JUNCTION_DEVIATION_MM. Please update your configuration."
 #elif defined(JUNCTION_ACCELERATION_FACTOR)
-  #error "JUNCTION_ACCELERATION_FACTOR is now JUNCTION_ACCELERATION. Please update your configuration."
+  #error "JUNCTION_ACCELERATION_FACTOR is obsolete. Delete it from Configuration_adv.h."
+#elif defined(JUNCTION_ACCELERATION)
+  #error "JUNCTION_ACCELERATION is obsolete. Delete it from Configuration_adv.h."
 #endif
 
 #define BOARD_MKS_13     -47

Marlin/src/module/planner.cpp

       }
       else {
         NOLESS(junction_cos_theta, -0.999999); // Check for numerical round-off to avoid divide by zero.
-        const float sin_theta_d2 = SQRT(0.5 * (1.0 - junction_cos_theta)); // Trig half angle identity. Always positive.
 
-        // TODO: Technically, the acceleration used in calculation needs to be limited by the minimum of the
-        // two junctions. However, this shouldn't be a significant problem except in extreme circumstances.
-        vmax_junction_sqr = (JUNCTION_ACCELERATION * JUNCTION_DEVIATION_MM * sin_theta_d2) / (1.0 - sin_theta_d2);
+        float junction_unit_vec[JD_AXES] = {
+          unit_vec[X_AXIS] - previous_unit_vec[X_AXIS],
+          unit_vec[Y_AXIS] - previous_unit_vec[Y_AXIS],
+          unit_vec[Z_AXIS] - previous_unit_vec[Z_AXIS]
+          #if ENABLED(JUNCTION_DEVIATION_INCLUDE_E)
+            , unit_vec[E_AXIS] - previous_unit_vec[E_AXIS]
+          #endif
+        };
+        // Convert delta vector to unit vector
+        normalize_junction_vector(junction_unit_vec);
+
+        const float junction_acceleration = limit_value_by_axis_maximum(block->acceleration, junction_unit_vec),
+                    sin_theta_d2 = SQRT(0.5 * (1.0 - junction_cos_theta)); // Trig half angle identity. Always positive.
+
+        vmax_junction_sqr = (junction_acceleration * JUNCTION_DEVIATION_MM * sin_theta_d2) / (1.0 - sin_theta_d2);
         if (block->millimeters < 1.0) {
 
           // Fast acos approximation, minus the error bar to be safe
 
           // If angle is greater than 135 degrees (octagon), find speed for approximate arc
           if (junction_theta > RADIANS(135)) {
-            const float limit_sqr = block->millimeters / (RADIANS(180) - junction_theta) * JUNCTION_ACCELERATION;
+            const float limit_sqr = block->millimeters / (RADIANS(180) - junction_theta) * junction_acceleration;
             NOMORE(vmax_junction_sqr, limit_sqr);
           }
         }

Marlin/src/module/planner.h

 
     static void recalculate();
 
+    #if ENABLED(JUNCTION_DEVIATION)
+
+      #if ENABLED(JUNCTION_DEVIATION_INCLUDE_E)
+        #define JD_AXES XYZE
+      #else
+        #define JD_AXES XYZ
+      #endif
+
+      FORCE_INLINE static void normalize_junction_vector(float (&vector)[JD_AXES]) {
+        float magnitude_sq = 0.0;
+        for (uint8_t idx = 0; idx < JD_AXES; idx++) if (vector[idx]) magnitude_sq += sq(vector[idx]);
+        const float inv_magnitude = 1.0 / SQRT(magnitude_sq);
+        for (uint8_t idx = 0; idx < JD_AXES; idx++) vector[idx] *= inv_magnitude;
+      }
+
+      FORCE_INLINE static float limit_value_by_axis_maximum(const float &max_value, float (&unit_vec)[JD_AXES]) {
+        float limit_value = max_value;
+        for (uint8_t idx = 0; idx < JD_AXES; idx++) if (unit_vec[idx]) // Avoid divide by zero
+          NOMORE(limit_value, ABS(max_acceleration_mm_per_s2[idx] / unit_vec[idx]));
+        return limit_value;
+      }
+
+    #endif // JUNCTION_DEVIATION
 };
 
 #define PLANNER_XY_FEEDRATE() (MIN(planner.max_feedrate_mm_s[X_AXIS], planner.max_feedrate_mm_s[Y_AXIS]))