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/GCodeWriter.hpp

@@ -72,7 +72,7 @@ public:
     // Prefix of the toolchange G-code line, to be used by the CoolingBuffer to separate sections of the G-code
     // printed with the same extruder.
     std::string toolchange_prefix() const;
-    std::string toolchange(unsigned int filament_id);
+    std::string toolchange(unsigned int filament_id, int nozzle_id);
     std::string set_speed(double F, const std::string &comment = std::string(), const std::string &cooling_marker = std::string());
     // SoftFever NOTE: the returned speed is mm/minute
     double      get_current_speed() const { return m_current_speed;}