Fix teccam loop: squeeze matrix before viridis broadcast

Mako.matrix returns (H,W,1) for mono cameras; passing it through
_apply_viridis produces (H,W,1,3) which array_to_png cannot unpack.
Squeeze to 2D in _run_loop and in STX Camera.matrix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>