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.

Art/blender/hero/hero-facade-2026-11-07.json

@@ -3,18 +3,18 @@
   "frame": "project frame, metres, X east Y north Z up",
   "bounds_m": {
     "min": [
-      -13.02,
+      -13.08,
       -12.5,
       0.0
     ],
     "max": [
-      13.02,
-      0.07,
+      13.08,
+      0.13,
       14.5
     ],
     "size": [
-      26.04,
-      12.57,
+      26.16,
+      12.63,
       14.5
     ]
   },
@@ -37,6 +37,24 @@
       ],
       "evidence": "6 columns at 4.96 m centres (sibling notes x measured frontage)"
     },
+    {
+      "object": "Hero_ColumnBase_x6",
+      "dims_m": [
+        1.36,
+        1.36,
+        0.22
+      ],
+      "evidence": "assumption: Ionic base moulding; photographs show a wider base at the shaft foot"
+    },
+    {
+      "object": "Hero_Abacus_x6",
+      "dims_m": [
+        1.3,
+        1.3,
+        0.14
+      ],
+      "evidence": "assumption: square abacus under the architrave, as Ionic capitals carry"
+    },
     {
       "object": "Hero_Architrave",
       "dims_m": [
@@ -86,10 +104,10 @@
       "object": "Hero_Medallion_x2",
       "dims_m": [
         1.1,
-        1.1,
-        0.02
+        0.04,
+        1.1
       ],
-      "evidence": "photographs: two stone discs beside the motto"
+      "evidence": "photographs: two stone discs beside the motto, mounted on the parapet face"
     },
     {
       "object": "Hero_Stylobate",