fix(dxf): stop writing a duplicate *Active viewport (#319)

Opening a DXF whose active viewport is stored uppercase (*ACTIVE),
editing and re-saving produced a file with two full-window model-space
viewports — the file's own *ACTIVE plus a fresh *Active we add on save.
The reserved viewport name is case-insensitive, so the two didn't
collapse and other CAD apps drew them one on top of the other, which
read as the copied objects being linked / "reacting as one".

save_model_tiles_to_vports dropped existing active viewports with a
case-sensitive `name != "*Active"` filter, so an uppercase *ACTIVE
survived and sat next to the new entry. Route every *Active comparison
(save filter + camera/tile restore) through a case-insensitive
is_active_vport_name helper so the file's record is recognised and
replaced instead of duplicated.

Also bumps acadrust to #99f75fe (merged PR #1): the ASCII DXF writer now
emits shortest round-trippable floats instead of the full 16-digit
expansion, halving the round-tripped file size.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

Cargo.lock

@@ -71,7 +71,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/HakanSeven12/acadrust?branch=main#f1f4dbf9f0e1273670556f86d3c26f2b14543fcc"
+source = "git+https://github.com/HakanSeven12/acadrust?branch=main#99f75fe116ba836ae5fab70a886290688bbc5f50"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",