feat(geo): adopt Mapbox building calibration (3.1 m per level)

Owner decision: adopt the two findings from the Mapbox cross-check.

- LEVEL_HEIGHT 3.2 -> 3.1 m in block_height_report.py and the Blosm
  --level-height default; every levels-derived building drops ~0.6 m per 6
  levels, matching Mapbox.
- HEIGHT_OVERRIDES for the three rear building=yes/apartments structures that
  Mapbox gives as single storey (3.0 m) where our 2-level fallback rendered
  6.4 m blocks.
- classify_height() takes an optional way_id so overrides apply; the Mapbox
  cross-check passes it and now reports 66 match / 6 non-building / 0 disagree.
- Geometry follows: blockout regenerated at 3.1 (frontage object names
  preserved, 6/6 exported), frontage re-imported, DemoMap rebuilt, packaged
  -qaselftest PASS (walk, reset, pause, transect 0 holes, wall probe 0/35).
- Docs: Art/blender/README scale table + a note that Blosm adds ~1.9 m roof mass
  above levels x levelHeight; TODO item closed; MEMORY records the gotcha that
  blockout regeneration must re-verify the element.* frontage mapping.

Art/blender/blockout/blosm_blockout.py

@@ -40,7 +40,9 @@ def parse_args():
     ap.add_argument("--osm", required=True, help="local .osm file to import")
     ap.add_argument("--bounds", required=True, help="south,west,north,east in WGS84 degrees")
     ap.add_argument("--out", required=True, help="output .blend path")
-    ap.add_argument("--level-height", type=float, default=3.2, help="metres per building level")
+    # 3.1 m, not 3.2: calibrated against Mapbox building data on 2026-11-09
+    # (see Geo/tools/mapbox_buildings.py and Geo/tools/block_height_report.py).
+    ap.add_argument("--level-height", type=float, default=3.1, help="metres per building level")
     ap.add_argument("--origin", default=None,
                     help="lat,lon to use as the Blender origin (project origin). Default: Blosm centres on the extract")
     ap.add_argument("--no-subdivide", action="store_true", help="skip OSM area subdivision")