blocks: XCLIP and XCLIPFRAME, clipped references kept through DXF and DWG

XCLIP clips block and xref references to a rectangular, polygonal or
closed-polyline boundary (Invert clip, ON/OFF, Delete, generate Polyline;
Clipdepth is ignored in 2D). The boundary is Insert::clip, in block
coordinates, so it follows its reference; other references stay whole.
Clipped references are drawn, plotted and framed (XCLIPFRAME 0/1/2) cut
to the boundary, attributes included, and their extents are cut to it.
DXF and DWG read and write it as the reference's ACAD_FILTER/SPATIAL
SPATIAL_FILTER; filters the model can't hold stay foreign data. Gap F2.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/engine/src/cmd/array.rs

@@ -154,6 +154,7 @@ pub(crate) fn make_assoc(s: &mut Session, objs: &[Handle], base: Vec2, mut p: Ar
         rows: 1,
         col_spacing: 0.0,
         row_spacing: 0.0,
+        clip: None,
     }))
 }