feat(facades): 0.1.6 retail frontage first pass — Lee Drug and 6806-6820

Models the two retail buildings beside the hero from their own photograph rather
than the generic kit: bronze cornice, mural row, lettering band, stepped Deco
parapet and rounded corner tower on 6800 Lee Drug; burgundy fascia, pilasters,
white-framed windows and banded parapet on 6806-6820. Reference analysis written
before any geometry (Art/blender/retail/ref-analysis.md), every unmeasured
proportion carried into the part report as an assumption.

6822 Budget is deferred, not guessed: no photograph of it exists and the 6.0 x 5.5 m
sliver cannot be read out of the neighbour shot.

Pipeline fixes found on the way:
- render_screenshots.sh called ./Engine/... for the capture step while every
  commandlet used the absolute path; set -e killed the script at 127 *after* the
  maps were rebuilt, so screenshots silently stayed a day old.
- the facade kit applies to every Frontage_*/North_* actor, so it pasted its own
  language over the new modelled facades; it now skips them, and the trim stages as
  RetailTrim_*.
- pure bay arithmetic moved to Tools/_common.py so it is unit-testable without
  Blender (4 new tests, 82 total pass).

Known gaps recorded in Screenshots/README.md, not hidden: the parapet reads as a
blank band, murals and lettering do not resolve at review distance, the tower is a
plain cylinder, and the review camera crops the east end.

Tools/tests/test_retail_frontage.py

@@ -0,0 +1,32 @@
+"""Tests for the pure layout helper in Tools/retail_frontage_build.py.
+
+The builder itself needs Blender; `bay_centres` does not, so the bay rhythm the
+façade depends on is checkable without starting Blender or Unreal.
+"""
+
+import unittest
+
+from Tools._common import bay_centres
+
+
+class BayCentreTests(unittest.TestCase):
+    def test_centres_stay_inside_the_margins(self):
+        centres = bay_centres(-33.20, -14.37, margin=1.2)
+        self.assertGreaterEqual(min(centres), -33.20 + 1.2 - 1e-9)
+        self.assertLessEqual(max(centres), -14.37 - 1.2 + 1e-9)
+
+    def test_bays_are_evenly_spaced(self):
+        centres = bay_centres(-66.08, -33.07, margin=1.0)
+        steps = {round(b - a, 6) for a, b in zip(centres, centres[1:])}
+        self.assertEqual(len(steps), 1, f"uneven bay spacing: {steps}")
+
+    def test_bay_count_is_the_round_of_span_over_bay(self):
+        # 33.01 m frontage, 1.0 m margin each end -> 31.01 m -> 14 bays at 2.2 m.
+        self.assertEqual(len(bay_centres(-66.08, -33.07, margin=1.0)), 14)
+
+    def test_degenerate_frontage_returns_nothing(self):
+        self.assertEqual(bay_centres(0.0, 1.0, margin=1.0), [])
+
+
+if __name__ == "__main__":
+    unittest.main()