test(visual): a shadow-gap measurement, and the spawn height cleared

Tools/shadow_gap.py measures the vertical pixel gap between an object's lowest pixels and the
top of its shadow, as a fraction of the object's height, and calls it contact or floating. It
classifies a shadow by per-channel scaling - a shadow darkens every channel by the same
proportion, an object does not - which separates the two where an absolute delta cannot. Four
unit tests cover contact, floating, an empty box and the border median.

On the real spawn view it returns 'unknown': the crop contains two surfaces (pale wall behind,
dark road below) and one global background cannot classify both. Recorded as the next step, not
presented as working.

The spawn height is measured and cleared: tracing down from 30 m, the topmost surface under the
player is Z 366 and the capsule's feet are at 369 - 3 cm, the capsule skin. If the character
still reads as floating, the gap is between the rendered pavement and its collision surface, not
the spawn Z, and that is what to measure next.

TODO.md

@@ -26,7 +26,10 @@ Actionable task list for this project. This file is the single, central place fo
 - [x] Fix the character and car floating. **The measurement was taken at the wrong moment.** The foot probe ran in `BeginPlay`, while the pawn is still at its spawn height and falling, so every trace reported a surface far below it (`ground 366` against an actor at 561) and read as a float that was not there. The probe now runs **three seconds after spawn**, from the capsule's feet, on three channels, and names the actor it hits. Character: the player start at Z 600 left the soles 39 cm in the air (the capsule settles at 561 on this pavement); the start is now the measured 561. Car: the template's lowest geometry is exhaust, so grounding on the whole model put the tyres 4 cm above the origin - it is now grounded on its wheels, staged at Z 342 after the traced road height (366) disagreed with the visible road by ~25 cm.
 - [x] Fix the vehicle's material colour. The template's `car paint` is a legacy mix-shader graph that the FBX exporter cannot map, so `SM_Vehicle_Body` imported holding `WorldGridMaterial` and rendered black while Blender showed red. Legacy graphs are now converted to Principled before export (saturated colour wins over the grey diffuse next to it), and the staged body panel is assigned `M_Vehicle_HatchPaint`. Judge: `verdict=yes confidence=0.9`.
 - [x] Fix the missing vehicle glazing. The `windows` material was metallic 1.0 with roughness 0.0 - a black void that read as an empty cabin. Now tinted translucent (opacity 0.85, roughness 0.08); opacity 0.45 was tried first and made the glass invisible. The vision judge still returns `verdict=no` for the frame, disagreeing with what is visibly tinted glass; it is advisory, and the human verdict decides.
-- [A] Measure the shadow gap instead of arguing with the vision model. The judge now flags the character and car as floating in the spawn view, and its criterion is in the expectation ("a shadow more than a hand's width below an object means the object is floating"). Two readings are possible and both fit the pixels: the object really floats, or it stands on the raised pavement and the sun throws its shadow past the kerb onto the road. A shadow-gap measurement - lowest object pixels versus its shadow, in pixels - settles it; the judge cannot.
+- [A] Measure the shadow gap instead of arguing with the vision model. The judge now flags the character and car as floating in the spawn view, and its criterion is in the expectation ("a shadow more than a hand's width below an object means the object is floating"). Two readings are possible and both fit the pixels: the object really floats, or it stands on the raised pavement and the sun throws its shadow past the kerb onto the road. `Tools/shadow_gap.py` now does the measurement: object vs shadow in pixels, as a fraction of the object's height. It works on synthetic frames (tests pass) but returns `unknown` on the real spawn view, because the crop contains two surfaces - the pale wall behind and the dark road below - and a single global background cannot classify both. Next: per-region backgrounds, or crop tightly around the feet and their shadow only.
+
+**The spawn height is measured and is not the problem:** tracing down from 30 m above the player, the topmost surface is Z 366 and the capsule's feet sit at 369 - three centimetres, which is the capsule skin. If the character still looks like it floats, the gap is between the **rendered** pavement and its **collision** surface, not the spawn Z. That is the thing to measure next: the sidewalk mesh's rendered top versus the collider returned by the trace.
+
 - [ ] Implement enter/exit interaction.
 - [ ] Build a closed driving route with boundaries.
 - [ ] Tune basic handling to a drivable state.