geo: one tool directory, one unit boundary, per-asset provenance

Geo/tools/ is now every geographic script, with Geo/README.md documenting each
one; the Blender-side builders moved beside their outputs in Art/blender/*/ and
import their helpers from Geo/tools/osm_common.py.

That module gains to_unreal_local() and to_blender_local(): the only place in the
repository that multiplies metres by 100, so a second silent scale cannot be
introduced. block_height_report.py adds height provenance for the block — 3 of
15 buildings have no height data and render at a placeholder 6.4 m.
asset_metadata.py writes and audits <asset>.geo.json sidecars; four now exist
(hero-facade, terrain, street, blockout) and the audit recomputes each centre from
the origin, failing anything that disagrees.

Art/blender/terrain/terrain.geo.json

@@ -0,0 +1,31 @@
+{
+  "schema": 1,
+  "asset": "terrain",
+  "kind": "terrain",
+  "generated_utc": "2026-11-07T11:27:43+00:00",
+  "geographic": {
+    "crs": "EPSG:26911",
+    "source": "3DEP 1 m DEM window, Geo/manifests/2026-11-06-3dep-dem.md",
+    "centre_easting_m": 376375.0,
+    "centre_northing_m": 3774225.0,
+    "elevation_navd88_m": 115.911,
+    "origin": {
+      "easting_m": 376515.389,
+      "northing_m": 3774224.083,
+      "reference": "Geo/manifests/local-origin.md"
+    }
+  },
+  "unreal": {
+    "centre_cm": [
+      -14038.9,
+      -91.7,
+      0.0
+    ],
+    "conversion": "100 uu per metre, applied once in osm_common.to_unreal_local",
+    "axes": "X east, Y south (FBX preset flips north), Z up"
+  },
+  "dims_report": null,
+  "notes": [
+    "2 m grid, 191 x 106, building footprints cut out; Z relative to origin ground 115.911 m NAVD88"
+  ]
+}