Protected buffer works! Vast improvements to planner efficiency. Many things still broken with overhaul.

Development push. Lots still broken.

- Protected planner concept works! This is a critical precursor to
enabling feedrate overrides in allowing the planner buffer and the
stepper execution operate atomically. This is done through a
intermediary segment buffer.

- Still lots of work to be done, as this was a complete overhaul of the
planner and stepper subsystems. The code can be cleaned up quite a bit,
re-enabling some of the broken features like feed holds, and finishing
up some of the concepts

- Pushed some of the fixes from the master and edge branch to here, as
this will likely replace the edge branch when done.

planner.h

@@ -33,20 +33,20 @@
 typedef struct {
 
   // Fields used by the bresenham algorithm for tracing the line
+  // NOTE: Do not change any of these values once set. The stepper algorithm uses them to execute the block correctly.
   uint8_t direction_bits;            // The direction bit set for this block (refers to *_DIRECTION_BIT in config.h)
   int32_t steps[N_AXIS];             // Step count along each axis
-  int32_t step_event_count;          // The number of step events required to complete this block
+  int32_t step_event_count;          // The maximum step axis count and number of steps required to complete this block. 
 
   // Fields used by the motion planner to manage acceleration
+  float entry_speed_sqr;             // The current planned entry speed at block junction in (mm/min)^2
+  float max_entry_speed_sqr;         // Maximum allowable entry speed based on the minimum of junction limit and 
+                                     //   neighboring nominal speeds with overrides in (mm/min)^2
+  float max_junction_speed_sqr;      // Junction entry speed limit based on direction vectors in (mm/min)^2
   float nominal_speed_sqr;           // Axis-limit adjusted nominal speed for this block in (mm/min)^2
-  float entry_speed_sqr;             // Entry speed at previous-current block junction in (mm/min)^2
-  float max_entry_speed_sqr;         // Maximum allowable junction entry speed in (mm/min)^2
-  float acceleration;                // Axes-limit adjusted line acceleration in mm/min^2
-  float millimeters;                 // The total travel for this block to be executed in mm
+  float acceleration;                // Axis-limit adjusted line acceleration in mm/min^2
+  float millimeters;                 // The remaining distance for this block to be executed in mm
   
-  // Settings for the trapezoid generator
-//   int32_t decelerate_after;  // The index of the step event on which to start decelerating
-
 } plan_block_t;
       
 // Initialize the motion plan subsystem
@@ -66,7 +66,9 @@ plan_block_t *plan_get_current_block();
 
 plan_block_t *plan_get_block_by_index(uint8_t block_index);
 
-int32_t calculate_trapezoid_for_block(uint8_t block_index);
+float plan_calculate_velocity_profile(uint8_t block_index);
+
+// void plan_update_partial_block(uint8_t block_index, float millimeters_remaining, uint8_t is_decelerating);
 
 // Reset the planner position vector (in steps)
 void plan_sync_position();