feat(belt/calib): add Overhang temperature-tower model (selectable) (#48)

Belt printers can't slice a tall vertical temperature tower. This adds a
belt-specific temperature-tower model — a row of discrete, individually
engraved provini laid along the belt, each printed at one temperature via
custom per-layer M104. Each provino is an inverted-L overhang that stresses
print quality, so the operator reads the best temperature off overhang
quality rather than a continuous ramp.

It is offered as a "Test model" choice in the temperature calibration dialog
(mirroring the Cornering test's selector), so users keep Joe's counter-rotated
sectioned tower as "Standard" and can pick this one as "Overhang":
- Calib_Params::test_model (existing field) carries the choice.
- Temp_Calibration_Dlg gets a Standard/Overhang radio.
- Plater::calib_temp belt branch: test_model 0 -> _calib_temp_belt_sectioned
  (unchanged Standard path), 1 -> the discrete-provini Overhang path.

Assets: belt_temp_provino_unit.stl + belt_temp_tower_<start>_<end>.stl (6
ranges) + gen_belt_temp_tower.py (manifold engraving). Based on
belt/generic-calibrations. The Overhang path is HW-validated on the IdeaFormer
IR3 V2 (discrete M104 + engraved numbers); not re-validated since the rebase.

resources/calib/temperature_tower/gen_belt_temp_tower.py

@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+"""Belt temperature-tower asset generator (discrete-provini design).
+
+A vertical temperature tower cannot be sliced on a belt printer, so lay a row of
+DISCRETE provini (one per temperature) along the belt (designed Y) with a fixed
+surface gap. Each provino is the chevron+arc unit (belt_temp_provino_unit.stl,
+keel-first); its temperature is ENGRAVED upright into the 50 mm face — a raised
+number would be an unsupported overhang on the belt. The C++ calib_temp belt branch
+(Plater.cpp) injects one M104 per zone 70 layers INTO provino i:
+  print_z[i] = i * PITCH * cos(theta) + 70 * layer_height   (theta = 45)
+inside the body, not in the empty inter-provino gap (which has no sliced layers for
+the event to attach to). PITCH below is the shared geometry contract with that code —
+keep them in sync.
+
+Generates one STL per filament temp range used by Temp_Calibration_Dlg.
+"""
+import numpy as np, trimesh, os
+from matplotlib.textpath import TextPath
+from matplotlib.font_manager import FontProperties
+from shapely.geometry import Polygon as ShPoly
+from shapely.ops import unary_union
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+UNIT = os.path.join(HERE, 'belt_temp_provino_unit.stl')   # single provino, keel-first
+SURF_GAP = 25.0            # surface-to-surface gap between provini (mm) — user spec
+TEXT_H = 9.0
+TEXT_DEPTH = 0.8           # engraving depth (numbers are CUT into the face, not raised:
+                          # a raised number is an unsupported Y-overhang on the belt)
+TEXT_OVERSHOOT = 0.6       # extra height poking out of the face for a clean boolean cut
+
+# Temperature ranges (start, end) per filament family, 5 C step. File name encodes them.
+RANGES = [(230,190),(270,230),(250,230),(280,240),(240,210),(320,280)]
+
+unit = trimesh.load(UNIT)
+dY = unit.bounds[1,1] - unit.bounds[0,1]
+PITCH = dY + SURF_GAP                       # designed-Y pitch == C++ contract constant
+print(f"unit dY={dY:.2f}  PITCH={PITCH:.3f}  (C++ contract: print_z[i]=i*{PITCH:.3f}*cos45)")
+
+# 50 mm face normal (0,-1,1)/sqrt2 ; UPRIGHT basis u=+X det(+1) (verified non-mirrored)
+n = np.array([0,-1,1.])/np.sqrt(2)
+u = np.array([1,0,0.]); v = np.array([0,1,1.])/np.sqrt(2)
+R = np.column_stack([u,v,n])
+fn = unit.face_normals; fc = unit.triangles_center; fa = unit.area_faces
+sel = (fn@n) > 0.9
+face_c = (fc[sel]*fa[sel,None]).sum(0)/fa[sel].sum()
+
+def text_mesh(s):
+    tp = TextPath((0,0), s, size=TEXT_H, prop=FontProperties(family='DejaVu Sans'))
+    rings = [ShPoly(p) for p in tp.to_polygons() if len(p)>=3]
+    rings.sort(key=lambda r:r.area, reverse=True)
+    used=[False]*len(rings); parts=[]
+    for i,o in enumerate(rings):
+        if used[i]: continue
+        holes=[]
+        for j in range(i+1,len(rings)):
+            if not used[j] and o.contains(rings[j]): holes.append(rings[j].exterior.coords); used[j]=True
+        parts.append(ShPoly(o.exterior.coords,holes)); used[i]=True
+    poly = unary_union(parts)
+    geoms = list(poly.geoms) if poly.geom_type=='MultiPolygon' else [poly]
+    m = trimesh.util.concatenate([trimesh.creation.extrude_polygon(g,height=TEXT_DEPTH+TEXT_OVERSHOOT) for g in geoms])
+    c = m.bounds.mean(axis=0); m.apply_translation([-c[0],-c[1],0]); return m
+
+for t_start, t_end in RANGES:
+    temps = list(range(t_start, t_end-1, -5))
+    parts=[]
+    for i,T in enumerate(temps):
+        c = unit.copy(); c.apply_translation([0, i*PITCH, 0])
+        t = text_mesh(str(T)); M=np.eye(4); M[:3,:3]=R; t.apply_transform(M)
+        # place the text spanning from TEXT_DEPTH inside the face to TEXT_OVERSHOOT outside,
+        # then CUT it out of the provino (engrave) — no raised material, no Y-overhang.
+        t.apply_translation(face_c - n*TEXT_DEPTH + np.array([0,i*PITCH,0]))
+        c = trimesh.boolean.difference([c, t], engine='manifold')
+        parts.append(c)
+    asset = trimesh.util.concatenate(parts)
+    out = os.path.join(HERE, f"belt_temp_tower_{t_start}_{t_end}.stl")
+    asset.export(out)
+    dims = np.round(asset.bounds[1]-asset.bounds[0],1)
+    wt = all(p.is_watertight for p in parts)
+    print(f"  {t_start}->{t_end}: {len(temps)} zones  bbox={dims}  watertight={wt}  -> {os.path.basename(out)}")