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 <noreply@anthropic.com>

src/linetypes.rs

@@ -22,9 +22,10 @@ pub enum LtSegment {
     Space(f32),
     /// Draw a dot (zero-length element, rendered as a tiny mark).
     Dot,
-    /// Draw a CXF shape at the current pen position.
+    /// Draw a shape at the current pen position. Embedded SHAPE elements are
+    /// no longer rendered (the LFF font set ships no shape file).
     Shape {
-        /// Shape name — looked up in `scene::cxf`.
+        /// Shape name (retained for round-tripping; not drawn).
         name: String,
         /// X offset along the linetype direction (can be negative).
         x: f32,