feat(io): default DWG save to 2018; bump acadrust handle-counter fix

- Save As now defaults the version dropdown to DWG/DXF 2018 (latest)
  instead of the loaded file's version; the selected version is written
  as-is (removed the source-version fallback for version-locked data).
- Pin acadrust 55a2caa: allocate_handle respects header.handle_seed as a
  floor, fixing the corruption where opening a file and saving it
  silently overwrote existing XRECORD/DICTIONARY objects (handle reuse)
  and AutoCAD refused the result.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -71,7 +71,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/HakanSeven12/acadrust?branch=main#0831995f4b219b8ff7e7b1b52bd147760880ff13"
+source = "git+https://github.com/HakanSeven12/acadrust?branch=main#55a2caa3c9318876fe61a8836221d0b4d17a2f62"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",