Finish Fixes from Copilot Review (#39)

* fix: restore BuildVolume bounds when toggling belt mode

set_belt_printer() mutated m_bboxf when enabling but never restored
the original extents on disable or when switching infinite_y true->false,
leaving stale max.y/max.z values that broke collision and object_state
checks. Recompute m_bboxf from m_bed_shape + m_max_print_height at the
top of each call, then apply belt-specific adjustments on top.

Addresses Copilot review comment on PR #12998 (BuildVolume.cpp:196).

* chore: drop [BELT-DEBUG] to_machine_coords log to trace

Was emitting at warning level once per 0.2mm Z bucket during every belt
print export, polluting default user logs. Trace level matches the rest
of the belt diagnostics and is silent in production.

Addresses Copilot review comment on PR #12998 (BeltGCodeWriter.cpp:86).

* chore: drop [BELTRACE] make_perimeters/support logs to trace

Eight warning-level traces around make_perimeters and
generate_support_material were emitting on every call/exit during normal
slicing, cluttering default logs. They're concurrency-debug breadcrumbs
not user-facing diagnostics, so drop them to trace.

Addresses Copilot review comment on PR #12998 (PrintObject.cpp:438).

* perf: gate BeltSliceStrategy diagnostic bbox tracking behind compile flag

apply_to_trafo() walked every model vertex twice (once for min_z, once
for per-volume mesh/slicer bboxes) and emitted seven trace logs per
call. The bboxes and logs are diagnostic only; min_z is the load-bearing
output. Wrap the bbox accumulation, logging, and supporting headers in
SLIC3R_BELT_DIAGNOSTIC_LOG so production builds do the bare min_z scan.

Addresses Copilot review comment on PR #12998 (BeltSliceStrategy.cpp:95).

* fix: apply part_cooling_fan_min_pwm to first-layer plane fan crossings

apply_first_layer_plane_fan_eval emitted band-crossing M106 commands
through GCodeWriter::set_fan() without the per-printer PWM floor that
every other set_fan call in CoolingBuffer applies. On printers with a
non-zero part_cooling_fan_min_pwm, fans could fail to spin up at low
requested speeds near the belt surface.

Addresses Copilot review comment on PR #12998 (CoolingBuffer.cpp:1227).

src/libslic3r/GCode/CoolingBuffer.cpp

@@ -1091,6 +1091,10 @@ std::string CoolingBuffer::apply_first_layer_plane_fan_eval(
     std::string out;
     out.reserve(gcode.size() + 256);
 
+    // Match the PWM floor applied at every other set_fan call in this file so
+    // band-crossing M106 emissions start the fan reliably at low speeds.
+    const unsigned int part_cooling_fan_min_pwm = static_cast<unsigned int>(std::max(0, m_config.part_cooling_fan_min_pwm.value));
+
     // Track position in slicing-frame mm.  Seed from m_current_pos which the
     // CoolingBuffer keeps up-to-date across layers.
     Vec3d cur_pos_mm(m_current_pos[0], m_current_pos[1], m_current_pos[2]);
@@ -1221,7 +1225,7 @@ std::string CoolingBuffer::apply_first_layer_plane_fan_eval(
                         target_fan = pre_band_main_fan;
                 }
                 if (target_fan != current_main_fan) {
-                    out += GCodeWriter::set_fan(m_config.gcode_flavor, target_fan);
+                    out += GCodeWriter::set_fan(m_config.gcode_flavor, target_fan, part_cooling_fan_min_pwm);
                     current_main_fan = target_fan;
                     m_fan_speed = target_fan;
                     m_current_fan_speed = target_fan;