fix(engine): real-change-only toolchange ordinals and a single M1020 toolchange command

A dual-nozzle H2C print with support filament hangs at its first nozzle
switch. The emitted file shows the change-filament block's M620 O ordinal
jumping from O1 straight to O230, plus a duplicate "M1020 S<n>" toolchange
command right after every change block. Two causes, fixed together because
they interlock (the ordinal check keys off the same toolchange detection
that suppresses the duplicate):

- append_tcr incremented m_toolchange_count once per prime-tower visit
  (roughly once per layer), while the change-filament template only emits
  its M620 O{toolchange_count + 1} line on real filament changes. With 229
  change-less sparse tower layers below the first support layer, the first
  real change reported ordinal 230. The counter now advances only when the
  expanded change block really contains a toolchange command, and the
  placeholder exposes the upcoming change's ordinal (count + 1). The
  set_extruder path already counted per real change and is unchanged.

- toolchange_prefix() returned "M1020 S" for BBL printers, so the
  custom_gcode_changes_tool() dedup could never match the stock profiles'
  line-leading "T[next_filament_id] ..." commands and the writer's own
  toolchange was appended after every change block on dual-extruder
  machines. The prefix is now the plain "T" (the manual-filament-change tag
  branch stays first), and the M1020 form moved into GCodeWriter::toolchange()
  as an explicit branch that also carries the nozzle:
  "M1020 S<filament> H<nozzle>". The nozzle parameter is signed on purpose:
  the null-safe nozzle lookup legitimately yields -1, matching the stock
  templates' own H-1 convention.

The prefix change also lets the CoolingBuffer recognize the change blocks'
T commands as tool boundaries on BBL printers (its per-filament attribution
previously keyed off the duplicate M1020, or nothing at all on
single-extruder models); its existing out-of-range guard ignores
T1000-class machine commands.

Verification: full suites green (libslic3r 48998 assertions / 169 cases;
fff_print 692 / 65 including three new scenarios - writer emission per
printer kind, dedup + ordinal progression on sequential prints, and a
prime-tower regression scenario verified to fail against the old per-visit
counting). Byte gate: 18 of 20 fixtures bit-identical; the sequential repro
differs by exactly its 3 removed duplicate M1020 lines, deterministic
across two runs. Reslicing the field project that exposed the hang yields
M620 O1 followed by a gapless O2..O59 and zero duplicate M1020 lines.

Co-authored-by: songwei.li <songwei.li@bambulab.com>

src/libslic3r/GCode.cpp

@@ -927,9 +927,6 @@ static std::vector<Vec2d> get_path_of_change_filament(const Print& print)
             }
         }
 
-        //BBS: increase toolchange count
-        gcodegen.m_toolchange_count++;
-
         std::string toolchange_gcode_str;
 
         ZHopType z_hope_type = ZHopType(gcodegen.config().z_hop_types.get_at(gcodegen.get_filament_config_index((int)gcodegen.writer().filament()->id())));
@@ -1058,7 +1055,7 @@ static std::vector<Vec2d> get_path_of_change_filament(const Print& print)
 
                 config.set_key_value("max_layer_z", new ConfigOptionFloat(gcodegen.m_max_layer_z));
                 config.set_key_value("relative_e_axis", new ConfigOptionBool(full_config.use_relative_e_distances));
-                config.set_key_value("toolchange_count", new ConfigOptionInt((int) gcodegen.m_toolchange_count));
+                config.set_key_value("toolchange_count", new ConfigOptionInt((int) gcodegen.m_toolchange_count + 1));
                 // BBS: fan speed is useless placeholer now, but we don't remove it to avoid
                 // slicing error in old change_filament_gcode in old 3MF
                 config.set_key_value("fan_speed", new ConfigOptionInt((int) 0));
@@ -1180,7 +1177,10 @@ static std::vector<Vec2d> get_path_of_change_filament(const Print& print)
 
         std::string toolchange_command;
         if (tcr.priming || (new_filament_id >= 0 && gcodegen.writer().need_toolchange(new_filament_id)))
-            toolchange_command = gcodegen.writer().toolchange(new_filament_id);
+            // Orca: null-safe, layer-aware nozzle lookup — group_result may be null on
+            // non-multi-nozzle paths (the helper falls back to the extruder id).
+            toolchange_command = gcodegen.writer().toolchange(new_filament_id,
+                nozzle_id_for_gcode_placeholder(group_result, new_filament_id, new_extruder_id, m_layer_idx));
         if (!custom_gcode_changes_tool(toolchange_gcode_str, gcodegen.writer().toolchange_prefix(), new_filament_id))
             toolchange_gcode_str += toolchange_command;
         else {
@@ -1302,6 +1302,10 @@ static std::vector<Vec2d> get_path_of_change_filament(const Print& print)
         std::string tcr_gcode, tcr_escaped_gcode = gcodegen.placeholder_parser_process("tcr_rotated_gcode", tcr_rotated_gcode, new_filament_id, &config);
         unescape_string_cstyle(tcr_escaped_gcode, tcr_gcode);
         gcode += tcr_gcode;
+        // Count the toolchange only when the emitted block really changed the tool —
+        // tower visits without a filament change must not advance the ordinal.
+        if (custom_gcode_changes_tool(tcr_gcode, gcodegen.writer().toolchange_prefix(), new_filament_id))
+            gcodegen.m_toolchange_count++;
         check_add_eol(toolchange_gcode_str);
 
         // SoftFever: set new PA for new filament
@@ -8648,7 +8652,7 @@ std::string GCode::set_extruder(unsigned int new_filament_id, double print_z, bo
             m_pa_processor->resetPreviousPA(m_config.pressure_advance.get_at(new_filament_id));
         }
 
-        gcode += m_writer.toolchange(new_filament_id);
+        gcode += m_writer.toolchange(new_filament_id, new_extruder_id);
         if (Extruder *fil = m_writer.filament())
             fil->set_config_index((int)get_filament_config_index((int)fil->id()));
         return gcode;
@@ -8933,7 +8937,7 @@ std::string GCode::set_extruder(unsigned int new_filament_id, double print_z, bo
 
     //BBS: don't add T[next extruder] if there is no T cmd on filament change
      //We inform the writer about what is happening, but we may not use the resulting gcode.
-    std::string toolchange_command = m_writer.toolchange(new_filament_id);
+    std::string toolchange_command = m_writer.toolchange(new_filament_id, next_nozzle_id);
     if (Extruder *fil = m_writer.filament())
         fil->set_config_index((int)get_filament_config_index((int)fil->id()));
     if (!custom_gcode_changes_tool(toolchange_gcode_parsed, m_writer.toolchange_prefix(), new_filament_id))