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.

tests/pdf_export_text_check.rs

@@ -88,6 +88,7 @@ fn text_and_dim_reach_pdf_export() {
         .cloned()
         .map(|mut w| {
             w.wire.text_verts.clear();
+            w.wire.searchable_text.clear();
             w
         })
         .collect();