gameplay: walkable demo map, reset and pause

Tools/ue_build_demo_map.py builds /Game/Hollywood/DemoMap: sun, sky, the hero
facade on the building line, six site meshes imported with collision, and a
player start on the north sidewalk. Tools/ue_import_site.py imports the site
FBXs; terrain, road and sidewalks use complex-as-simple so a simple hull cannot
ramp over the kerbs. Art/blender/site/export_site_fbx.py exports one FBX per
object so each can carry its own collision.

Reset (R) and pause (Escape) join the character and the input context: reset
restores the spawn transform and clears momentum, pause is a real game pause that
releases the cursor. DefaultInput.ini captures the mouse on launch.

Three placement defects found by measuring the imported actors rather than
trusting the plan: the spawn was on the wrong sidewalk because the FBX flip
inverts north, a mirror I added to the exporter double-corrected an already
correct sidewalk, and the street meshes only carry part of the north walk - the
rest is terrain.

MEMORY.md

@@ -126,3 +126,12 @@ Scope: facts and traps that cost time to establish. Planned work lives in [ROADM
 - **Model bottom must land on Z 0.** The hero portal jambs were sized `PORTAL_H + 0.3` and centred at `PORTAL_H / 2`, so they hung 0.15 m below the pavement and the whole facade imported with `min z = -15 cm`. Always check the asset's `bounds_m.min.z` in the build report.
 - **Rebuilding a facade is not enough to make a detail shot work**: doors modelled as thin panels on a solid plinth read as shading, not openings. Cutting the plinth into piers with a lintel band is what makes them legible from 8 m.
 
+## 2026-11-07 — Demo map assembly traps
+
+- **Unreal signs, measured not assumed.** The FBX preset flips north, so in the demo map the road spans Y -1382..+1047, the **north** sidewalk Y -1912..-1053, the **south** sidewalk Y +718..+1447, and the facade sits at Y -1450. A PlayerStart at +1600 is on the south sidewalk behind the building. Read the actor bounds before placing anything.
+- **Street meshes carry only PART of the sidewalk.** In the source `.blend`, `Sidewalk_South` is the offset band (-14.47..-7.18 m) and `Sidewalk_North` is the kerb-to-wall band (10.53..19.12 m); the rest of the north walk comes from the terrain. So a sidewalk-looking strip of ground between kerb and wall is the terrain, not a gap.
+- **Do not mirror in the exporter.** `Sidewalk_South` was already correct in the `.blend`; mirroring both sidewalks at export moved the correct one to the wrong side and produced an "everything on the wrong side" look that took three rebuilds to unpick. Check the source coordinates first.
+- **`bpy.ops.mesh.normals_make_consistent` needs a mesh context**; in object mode it fails with `poll() failed, context is incorrect`. Flip winding by reversing `polygon.vertices` instead.
+- **`StaticMesh` has no trace-flag property in Python.** Collision complexity is set through `mesh.get_editor_property("body_setup").set_editor_property("collision_trace_flag", …)`; `StaticMeshEditorSubsystem.get_collision_complexity` reads it back but there is no setter.
+- **A `-game` run with a paused world still exits on timeout**, so a smoke test that "hangs" may simply be running.
+