feat(pdf): searchable text layer with per-style subset font embedding

Real BT/Tj text objects ride alongside the vector outlines (Tr 3,
zero visual change): runs preserved at layout time, TTF styles embedded
as allsorts subsets with ToUnicode, width-matched Tm, clip-culled.
WireModel literals moved to ..Default::default() tails.

Cargo.lock

@@ -6,6 +6,7 @@ version = 4
 name = "OpenCADStudio"
 version = "2026.39.0"
 dependencies = [
+ "allsorts",
  "ashpd",
  "base64",
  "bincode",