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/BuildVolume.cpp

@@ -182,6 +182,12 @@ void BuildVolume::set_belt_printer(bool enabled, double angle_deg, bool infinite
     m_belt_angle = angle_deg;
     m_belt_infinite_y = infinite_y;
 
+    // Restart from the unmodified bbox each call. Without this, toggling
+    // belt mode off (or switching infinite_y true→false) would leave the
+    // extents inflated and break collision / object_state checks.
+    BoundingBoxf bboxf = get_extents(m_bed_shape);
+    m_bboxf = BoundingBoxf3{ to_3d(bboxf.min, 0.), to_3d(bboxf.max, m_max_print_height) };
+
     if (enabled) {
         if (infinite_y) {
             // Extend the Y bound to a very large value for infinite belt.