feat(fonts): switch the text engine from CXF to LibreCAD LFF fonts

Replace the small QCAD CXF stroke-font set with the full LibreCAD LFF font
library (46 fonts) and a new LFF parser/engine in src/scene/lff.rs. The
parser reads the `[hex] char` glyph blocks, tessellates `,A<bulge>` arcs, and
resolves `C<hex>` glyph references. Per LibreCAD's RS_Polyline convention a
vertex's bulge curves the segment ending at that vertex — getting this wrong
turned round letters like O into sideways capsules.

Font name resolution mirrors LibreCAD: match by file stem, map common
AutoCAD/DXF SHX names (txt, isocp, complex, italic, gothic*, gost, …) to the
nearest LFF family, and fall back to `standard`. A missing glyph in the chosen
family falls back to the broad `unicode` font so accented/Cyrillic/Greek text
still renders. Cap height stays 9 units, so the text scale is unchanged.

All text consumers move from cxf:: to lff::; complex linetypes still render
embedded text but no longer draw embedded shapes (LibreCAD ships no shape
file). The CXF module and every *.cxf asset are removed, and the MText / text
style font pickers list the LFF families.

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

src/ui/textstyle.rs

@@ -60,26 +60,26 @@ const LIST: Color = Color {
 };
 
 const BUILTIN_FONTS: &[&str] = &[
-    "CourierCad.cxf",
-    "Cursive.cxf",
-    "GothGBT.cxf",
-    "GothGRT.cxf",
-    "GothITT.cxf",
-    "GreekC.cxf",
-    "GreekS.cxf",
-    "ItalicC.cxf",
-    "ItalicT.cxf",
-    "RomanC.cxf",
-    "RomanD.cxf",
-    "RomanS.cxf",
-    "RomanT.cxf",
-    "SansND.cxf",
-    "SansNS.cxf",
-    "ScriptC.cxf",
-    "ScriptS.cxf",
-    "Standard.cxf",
-    "Unicode.cxf",
-    "SymbolCad.cxf",
+    "Standard",
+    "ISO",
+    "Simplex",
+    "RomanS",
+    "RomanD",
+    "RomanC",
+    "RomanT",
+    "ItalicC",
+    "ItalicT",
+    "ScriptS",
+    "ScriptC",
+    "GothGBT",
+    "GothGRT",
+    "GothITT",
+    "Cursive",
+    "GreekC",
+    "Symbol",
+    "ISO",
+    "ISO3098",
+    "Unicode",
 ];
 
 fn btn_s(accent: bool) -> impl Fn(&Theme, button::Status) -> button::Style {