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/_common.py

@@ -0,0 +1,27 @@
+"""Shared helpers that must stay importable outside Blender and Unreal.
+
+Blender-side builders import `bpy` at module scope, so nothing in them can be unit
+tested. Pure layout arithmetic lives here instead: the builders import it, and
+`Tools/tests/` imports it without a 3D application.
+
+Standard library only.
+"""
+
+from __future__ import annotations
+
+
+def bay_centres(x0, x1, margin=0.9, bay=2.2):
+    """Centres of equal bays across a frontage, inset by `margin` at each end.
+
+    Used by the façade kit and the retail frontage so both use one rhythm. The count
+    is the span divided by the bay module and rounded, so a 33 m frontage at a 2.2 m
+    module gets 14 bays rather than one bay per 2.2 m of wall with a stub at the end.
+
+    Returns [] when the margins consume the whole frontage.
+    """
+    span = (x1 - x0) - 2.0 * margin
+    if span <= 0:
+        return []
+    count = max(1, int(round(span / bay)))
+    step = span / count
+    return [x0 + margin + step * (i + 0.5) for i in range(count)]