Two layout-related fixes:
1. Sheet viewport (id=1) was rendered as a grey rectangle border in
paper-space layouts. It is the paper boundary, not a user viewport,
and should not appear. Now filtered via is_content_viewport() before
tessellation. The redundant id==1 color branch is removed.
2. Box crossing-selection (right-to-left drag) only detected wires whose
vertex points fell inside the selection box. Large entities like viewport
frames could be visually crossed without any corner inside the box and
would not be selected. Added segment-box edge intersection tests so any
wire segment that crosses the selection rectangle boundary is included.
Fixes part of #10
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>