…rate cells

Two independent crashes in CreateDrawing, the second of which was masked by
the first.

Treat an unreadable linework cache as a cache miss (IfcOpenShell#9309). A cache file
being present on disk does not mean it is usable: it may be zero bytes or
truncated from an interrupted write, or an unhydrated cloud storage
placeholder whose sync provider isn't running, in which case the directory
entry reports the full logical size but no content can be read back.
generate_linework checked os.path.isfile() and treated that as proof of
readability, so etree.parse() aborted the whole print operation. Add
parse_cached_svg(), which returns None and reports a warning instead of
raising, and use it at both call sites. The cache is derived data, so
regenerating is always safe. The early return needed the guard too, since
combine_svgs reads that path back line by line and cannot recover from it.

Skip degenerate cell polygons (IfcOpenShell#9310). shapely.Polygon() rejects a 1 or 2
coordinate ring, and did so before the existing `polygon.area < 1` check
could discard the sliver. Filter on coordinate count first. This is the same
defect fixed in IfcOpenShell#2836 for merge_linework_and_add_metadata, which is already
guarded by `len(coords) > 2`; the cell path loop never got the equivalent.

Verified valid polygons are unaffected: triangles and squares are still
kept, and sub-1mm2 polygons are still dropped by the area check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>