fix: stop drawing pose landmarks the model never saw

[Endika]

From a screenshot of the app running against a signer framed from the chest up: the torso box ran off the bottom of the picture onto the desk, the arms pointed at the frame edges instead of following the arms, and the neck was a wide triangle across the chest.

Root cause

MediaPipe extrapolates pose landmarks it cannot see rather than omitting them. Film someone from the chest up and it still returns hip coordinates — invented, somewhere below the picture. It marks them with a low visibility, which the overlay was ignoring entirely.

Now every pose edge requires both endpoints above MIN_POSE_VISIBILITY. The shoulder line stays, the hip box disappears when the hips are out of shot, and arms are drawn only where they were actually found.

The neck was my shortcut, not the model's fault

I had drawn it as nose→left-shoulder plus nose→right-shoulder, because MediaPipe has no neck landmark. That paints a triangle over the chest — over exactly the signing space the user needs to see. It is now one segment from the shoulder midpoint up to roughly the chin, stopping short of the nose so it does not run into the face mesh.

Chips now mean what they say

They reported pose.length > 0 — "the pose model returned an array" — so torso and arms went green even when nothing about them was visible. Each chip is now driven by the visibility of the landmarks that part is actually made of, so red means not seen.

Nine tests, including that absent visibility is trusted rather than treated as invisible: hands and face do not report it, and getting that backwards would erase them from the overlay.