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/entities/attribute.rs

@@ -15,7 +15,7 @@ use crate::entities::traits::{Grippable, PropertyEditable, Transformable, TruckC
 use crate::scene::acad_to_truck::{TextStroke, TruckEntity, TruckObject};
 use crate::scene::object::{GripApply, GripDef, PropSection, PropValue, Property};
 use crate::scene::wire_model::SnapHint;
-use crate::scene::{cxf, transform};
+use crate::scene::{lff, transform};
 
 // ── Shared helpers ────────────────────────────────────────────────────────────
 
@@ -273,7 +273,7 @@ fn build_attr_truck(input: AttrTextInputs<'_>, document: &acadrust::CadDocument)
             anchor_f64[0] - (anchor_local_x as f64 * cos_r - local_y_for_line as f64 * sin_r),
             anchor_f64[1] - (anchor_local_x as f64 * sin_r + local_y_for_line as f64 * cos_r),
         ];
-        let strokes = cxf::tessellate_text_ex(
+        let strokes = lff::tessellate_text_ex(
             [0.0, 0.0],
             input.height as f32,
             rotation,