Make the wipe tower's planner flush and dwell work on Klipper (#15133)

# Description

On Klipper the wipe tower's motion-queue synchronization silently did
nothing. Klipper acts on commands the moment it parses them, and its
`G4` reads only `P` in milliseconds — it ignores `S` — so the `G4 S0`
the tower used to flush the queue before a temperature change never
synchronized anything, and the cooling delay after a filament's cooling
moves passed instantly instead of waiting. The tower now emits `M400`
for the flush and `G4 P<ms>` for the dwell when the flavor is Klipper.

Only `gcode_flavor = klipper` is affected; G-code for every other flavor
is byte-identical, so no shipped profile or existing project file
changes.

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src/libslic3r/GCode/WipeTower.cpp

@@ -617,6 +617,18 @@ Polygon generate_rectange_polygon(const Vec2f &wt_box_min ,const Vec2f & wt_box_
     return res;
 }
 
+const char* flush_planner_queue_command(GCodeFlavor flavor)
+{
+    return flavor == gcfKlipper ? "M400\n" : "G4 S0\n";
+}
+
+std::string wait_command(GCodeFlavor flavor, float seconds)
+{
+    if (flavor == gcfKlipper)
+        return "G4 P" + std::to_string(std::lround(seconds * 1000.f)) + "\n";
+    return "G4 S" + Slic3r::float_to_string_decimal_point(seconds, 3) + "\n";
+}
+
 class WipeTowerWriter
 {
 public:
@@ -1145,7 +1157,7 @@ public:
 	{
         if (time==0.f)
             return *this;
-        m_gcode += "G4 S" + Slic3r::float_to_string_decimal_point(time, 3) + "\n";
+        m_gcode += wait_command(m_gcode_flavor, time);
 		return *this;
     }
 
@@ -1190,7 +1202,7 @@ public:
 
 	WipeTowerWriter& flush_planner_queue()
 	{
-		m_gcode += "G4 S0\n";
+		m_gcode += flush_planner_queue_command(m_gcode_flavor);
 		return *this;
 	}
 
@@ -1333,6 +1345,8 @@ public:
     {
         std::string buffer;
         if (wait_for_moves)
+            // Not flush_planner_queue_command(): this BBL precool path wants M400, which every
+            // flavor it reaches understands, not the zero dwell the other flavors flush with.
             buffer += "M400\n";
         buffer += "M104";
         if (target_extruder != -1)

src/libslic3r/GCode/WipeTower.hpp

@@ -26,6 +26,12 @@ enum GCodeFlavor : unsigned char;
 Polylines construct_gap_for_skip_points(
     const Polygon& polygon, const std::vector<Vec2f>& skip_points, float wt_width, float gap_length, Polygon& insert_skip_polygon);
 
+// Klipper acts on commands the instant it parses them, and its G4 reads only P (milliseconds),
+// so the zero-second and seconds-valued dwells every other flavor uses neither synchronize nor
+// pause there. Both defined in WipeTower.cpp, shared by WipeTower and WipeTower2.
+const char* flush_planner_queue_command(GCodeFlavor flavor); // finish queued moves, e.g. around M104/M109
+std::string wait_command(GCodeFlavor flavor, float seconds);  // pause for `seconds`
+
 class WipeTower
 {
 public:

src/libslic3r/GCode/WipeTower2.cpp

@@ -386,7 +386,8 @@ public:
     }
 
     WipeTowerWriter2& switch_filament_monitoring(bool enable) {
-        m_gcode += std::string("G4 S0\n") + "M591 " + (enable ? "R" : "S0") + "\n";
+        flush_planner_queue();
+        m_gcode += enable ? "M591 R\n" : "M591 S0\n";
         return *this;
     }
 
@@ -625,7 +626,7 @@ public:
 	// Set extruder temperature, don't wait by default.
 	WipeTowerWriter2& set_extruder_temp(int temperature, bool wait = false)
 	{
-        m_gcode += "G4 S0\n"; // to flush planner queue
+        flush_planner_queue();
         m_gcode += "M" + std::to_string(wait ? 109 : 104) + " S" + std::to_string(temperature) + "\n";
         return *this;
     }
@@ -635,7 +636,7 @@ public:
 	{
         if (time==0.f)
             return *this;
-        m_gcode += "G4 S" + Slic3r::float_to_string_decimal_point(time, 3) + "\n";
+        m_gcode += wait_command(m_gcode_flavor, time);
 		return *this;
     }
 
@@ -677,8 +678,8 @@ public:
     }
 
 	WipeTowerWriter2& flush_planner_queue()
-	{ 
-		m_gcode += "G4 S0\n"; 
+	{
+		m_gcode += flush_planner_queue_command(m_gcode_flavor);
 		return *this;
 	}