feat(hero): Ionic base and abacus, medallions onto the parapet face

Detail pass on the hero, all verified by measuring the built mesh:

- Column base moulding: a 1.36 m wide, 0.22 m tall cylinder at each shaft foot
  (2.00-2.22 m). The shaft previously started straight off the square pedestal,
  so the base never read in silhouette.
- Abacus: a 1.30 m square, 0.14 m thick slab over each capital. It takes the top
  of the capital zone, so the colonnade still ends at 9.70 m and the 53.1 %
  proportion is unchanged.
- Medallions fixed: they were built with the Z-axis cylinder helper, so the two
  'discs' were pancakes lying flat on the parapet coping (1.1 x 1.1 x 0.02,
  z 14.02-14.04). New disc_on_wall() helper rotates the cylinder 90 deg about X;
  they now stand 1.1 m across on the parapet face (y -1.32..-1.28,
  z 13.05..14.15).

Hero rebuilt (95 parts, 2420 tris, still 14.50 m), re-imported, DemoMap rebuilt,
screenshot set re-rendered. Part report records the new dims and their assumption
notes. Screenshots README notes the detail pass.

Tools/hero_facade_build.py

@@ -88,6 +88,25 @@ def cylinder(name, x, y, z0, z1, radius, material, vertices=24):
     return obj
 
 
+def disc_on_wall(name, x, y, z, radius, thickness, material=None, vertices=24):
+    """A round disc mounted vertically on a wall face.
+
+    `cylinder()` builds along Z, so a "medallion" made with it is a pancake lying
+    flat on top of the parapet rather than a disc on its face — which is what the
+    first build did (1.1 m across in Y, 0.02 m thick in Z, sitting at the coping).
+    This rotates the cylinder 90 degrees about X so the axis points along Y and
+    the disc faces the street.
+    """
+    obj = cylinder(name, x, y, z - thickness / 2.0, z + thickness / 2.0,
+                   radius, material, vertices=vertices)
+    obj.rotation_euler = (math.radians(90.0), 0.0, 0.0)
+    bpy.ops.object.select_all(action="DESELECT")
+    obj.select_set(True)
+    bpy.context.view_layer.objects.active = obj
+    bpy.ops.object.transform_apply(location=False, rotation=True, scale=True)
+    return obj
+
+
 def make_material(name, rgba, roughness=0.75):
     mat = bpy.data.materials.new(name)
     mat.use_nodes = True
@@ -136,14 +155,32 @@ def build():
         col_xs.append(x)
         cube(f"Hero_Pedestal_{i + 1}", x, -0.62, PEDESTAL_H + 0.27,
              1.24, 1.24, 0.54, stone)
-        cylinder(f"Hero_Column_{i + 1}", x, -0.55, PEDESTAL_H + 0.55,
-                 PEDESTAL_H + 0.55 + (HEIGHT - ENTABLATURE_H - PARAPET_H - PEDESTAL_H - 0.55 - CAPITAL_H),
+        # Base moulding: a taller, wider cylinder where the shaft meets the
+        # pedestal. Without it the shaft starts straight off a square plinth,
+        # which is not how an Ionic base reads in silhouette.
+        base_z0 = PEDESTAL_H + 0.55
+        base_h = 0.22
+        cylinder(f"Hero_ColumnBase_{i + 1}", x, -0.55, base_z0, base_z0 + base_h,
+                 0.68, stone, vertices=20)
+        shaft_z0 = base_z0 + base_h
+        cap_z0 = HEIGHT - ENTABLATURE_H - PARAPET_H - CAPITAL_H
+        cylinder(f"Hero_Column_{i + 1}", x, -0.55, shaft_z0, cap_z0,
                  COLUMN_D / 2.0, stone)
-        cylinder(f"Hero_Capital_{i + 1}", x, -0.55,
-                 HEIGHT - ENTABLATURE_H - PARAPET_H - CAPITAL_H,
-                 HEIGHT - ENTABLATURE_H - PARAPET_H, 0.62, stone, vertices=16)
+        # Capital block, then the square abacus that carries the architrave.
+        # The abacus takes the top 0.14 m of the capital zone so the colonnade
+        # still ends at HEIGHT - ENTABLATURE_H - PARAPET_H (proportion unchanged).
+        abacus_h = 0.14
+        cylinder(f"Hero_Capital_{i + 1}", x, -0.55, cap_z0,
+                 HEIGHT - ENTABLATURE_H - PARAPET_H - abacus_h, 0.62, stone, vertices=16)
+        cube(f"Hero_Abacus_{i + 1}", x, -0.55,
+             HEIGHT - ENTABLATURE_H - PARAPET_H - abacus_h / 2.0,
+             1.30, 1.30, abacus_h, stone)
     note("Hero_Column_x6", (COLUMN_D, COLUMN_D, HEIGHT - ENTABLATURE_H - PARAPET_H - PEDESTAL_H),
          f"6 columns at {COLUMN_BAY} m centres (sibling notes x measured frontage)")
+    note("Hero_ColumnBase_x6", (1.36, 1.36, 0.22),
+         "assumption: Ionic base moulding; photographs show a wider base at the shaft foot")
+    note("Hero_Abacus_x6", (1.30, 1.30, 0.14),
+         "assumption: square abacus under the architrave, as Ionic capitals carry")
 
     # --- entablature and parapet ------------------------------------------
     ent_z0 = HEIGHT - ENTABLATURE_H - PARAPET_H
@@ -167,11 +204,13 @@ def build():
          FRONTAGE_W - 3.0, 0.08, 0.85, stone_dark)
     note("Hero_MottoBand", (FRONTAGE_W - 3.0, 0.08, 0.85), "photographs: motto panel above the colonnade")
 
-    # Medallions either side of the inscription.
+    # Medallions either side of the inscription: discs on the parapet face, not
+    # pancakes on the coping (see disc_on_wall).
     for i, x in enumerate((-6.2, 6.2), start=1):
-        cylinder(f"Hero_Medallion_{i}", x, -1.36, HEIGHT - PARAPET_H / 2.0 + 0.42,
-                 HEIGHT - PARAPET_H / 2.0 + 0.44, 0.55, stone_dark, vertices=20)
-    note("Hero_Medallion_x2", (1.1, 1.1, 0.02), "photographs: two stone discs beside the motto")
+        disc_on_wall(f"Hero_Medallion_{i}", x, -1.30, HEIGHT - PARAPET_H / 2.0,
+                     0.55, 0.04, stone_dark, vertices=20)
+    note("Hero_Medallion_x2", (1.1, 0.04, 1.1),
+         "photographs: two stone discs beside the motto, mounted on the parapet face")
 
     # --- entrances ---------------------------------------------------------
     # Three portals between column pairs, each with its own short flight. The