Robot descriptions v2.16.0

Shared color palette

Edit source/appearance/colors.json to change colors across PAR6, PAROL6,
standalone MSG and standalone SSG48 in URDF and MuJoCo. Common structure/motor
colors, individual-part overrides, all jaw colors, pneumatic colors and the
PAR6 badge palette are supported.

Run python tools/build.py --models-only --render after an appearance edit.
The build applies the palette, rebakes the badge, refreshes review snapshots,
validates the models and exports all eight packages. OpenGL is needed only for
refreshing screenshots. Each package carries its applied palette in
docs/colors.json; model loading requires no theme parser or extra process.

The initial palette preserves the existing look. Geometry, collision surfaces,
kinematics, masses, COM, inertia, actuation and cameras retain their definitions.
Non-default-theme regression checks verify all compiled physical/mesh arrays
and all URDF non-visual definitions stay unchanged.

Models/assets/documentation remain CC BY 4.0; support code stays Apache-2.0.
Hardware accuracy/calibration work remains deferred.