feat(hero): fluting and a recessed motto band; placement gets one source of truth

Hero detail:
- Fluting: 20 grooves per column as thin shadow-tone cylinders set into the shaft
  (120 pieces) rather than boolean cuts - readable in silhouette and grazing light
  at a fraction of the cost.
- Motto band: real recess with a moulded surround 6 cm proud and the panel 12 cm
  behind, replacing the 8 cm dark stripe.
  Hero 148 parts/3044 tris -> 271/5492, still 14.50 m.

Placement bug this exposed (a real one, invisible in a render):
- ue_hero_facade_import.py carried its own hero position (-12024.5, -1450) from
  before the 0.1.0 frontage fix, so running the importer alone moved the hero 13 m
  east and onto the north side of the boulevard. Only symptom: the reference score
  fell 0.69 -> 0.56 and the 3x3 composition correlation flipped +0.54 -> -0.08.
- New Tools/ue_placement.py is the single source of truth (hero position, verified
  frontage spans, span validators). Both the importer and the demo builder use it;
  the import asserts the staged span and logs PLACEMENT ERROR otherwise.
- 11 new tests, including one that fails if the stale literal returns. 78 total.

Spawn-view score recovered to 0.72 (best yet); screenshots verified 4/4, packaged QA
all PASS.

Tools/hero_facade_build.py

@@ -174,6 +174,20 @@ def build():
         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)
+
+        # Fluting: 20 shallow grooves around the shaft, as the photographs show on
+        # the colonnade. Each flute is a thin cylinder set into the shaft surface;
+        # modelling them as grooves would need booleans on 6 columns for detail the
+        # eye reads only in silhouette and grazing light, so they are added mass in
+        # the *shadow* direction instead — cheap, and it breaks the flat highlight.
+        flutes = 20
+        flute_r = 0.032
+        for f in range(flutes):
+            ang = (2.0 * math.pi * f) / flutes
+            fx = x + (COLUMN_D / 2.0 - flute_r * 0.4) * math.cos(ang)
+            fy = -0.55 + (COLUMN_D / 2.0 - flute_r * 0.4) * math.sin(ang)
+            cylinder(f"Hero_Flute_{i + 1}_{f + 1}", fx, fy, shaft_z0 + 0.05,
+                     cap_z0 - 0.05, flute_r, stone_dark, vertices=6)
         # 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).
@@ -185,6 +199,9 @@ def build():
              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_Flute_x120", (0.064, 0.064, cap_z0 - shaft_z0),
+         "photographs: fluted shafts; 20 grooves per column modelled as shallow "
+         "cylinders in the shadow tone rather than boolean cut grooves")
     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),
@@ -223,10 +240,22 @@ def build():
          FRONTAGE_W, 1.2, PARAPET_H, stone)
     note("Hero_Parapet", (FRONTAGE_W, 1.2, PARAPET_H), "assumption: parapet carries the motto band")
 
-    # Motto band: a recessed panel across the parapet, as the photographs show.
-    cube("Hero_MottoBand", 0.0, -1.32, HEIGHT - PARAPET_H / 2.0,
-         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")
+    # Motto band: a recessed panel across the parapet with a moulded surround, as
+    # the photographs show. The panel was a 8 cm stripe of dark stone; it is now a
+    # real recess — surround frame standing 6 cm proud, panel set 12 cm behind it —
+    # so it reads as an inscription panel from the street instead of a paint line.
+    motto_z = HEIGHT - PARAPET_H / 2.0
+    motto_w = FRONTAGE_W - 3.0
+    for x_off in (-(motto_w / 2.0 + 0.16), (motto_w / 2.0 + 0.16)):
+        cube(f"Hero_MottoSurround_v{int(x_off > 0)}", x_off, -1.13, motto_z,
+             0.32, 0.26, 1.15, stone)
+    for z_off in (-0.575, 0.575):
+        cube(f"Hero_MottoSurround_h{int(z_off > 0)}", 0.0, -1.13, motto_z + z_off,
+             motto_w + 0.64, 0.26, 0.32, stone)
+    cube("Hero_MottoBand", 0.0, -1.06, motto_z, motto_w, 0.10, 0.85, stone_dark)
+    note("Hero_MottoBand", (motto_w, 0.10, 0.85),
+         "photographs: motto panel above the colonnade, now an inset panel with a "
+         "moulded surround rather than a flat stripe")
 
     # Medallions either side of the inscription: discs on the parapet face, not
     # pancakes on the coping (see disc_on_wall).