fix(scene): correct MIRROR of TEXT under MIRRTEXT (commit + preview)

Root cause: the glyph tessellator advanced the pen by advance*width_factor
while the transform already scales the pen position by the width factor, so
a backward (negative width factor) run kept its word order and grew on the
same side of the anchor instead of mirroring across it. Advance in unscaled
glyph units to match text_local_bounds; the transform applies the factor once.

On top of that:
- The renderer honours the entity's own text-generation flags (DXF 71: bit 2
  backward / bit 4 upside-down) XOR the text style's flags, and signs the
  horizontal anchor by the width factor so Center/Right backward text mirrors
  about its anchor.
- MIRRTEXT on: a MIRROR toggles both group-71 bits so the reflected rotation
  renders a true glyph mirror (backward-upright for a vertical axis, flipped
  for a horizontal one).
- MIRRTEXT off: the text flips left<->right and lands mirror-symmetric to the
  source (right-reading) instead of hugging the axis.
- The MIRROR preview re-derives text ghosts to match the commit; mirrored()
  now reflects the double-single residual too.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/text.rs

@@ -69,14 +69,15 @@ fn to_truck(t: &Text, document: &acadrust::CadDocument) -> TruckEntity {
     } else {
         resolved_style.width_factor.max(0.01)
     };
-    // is_backward mirrors text left-right via negative width factor.
-    let width_factor = if resolved_style.is_backward {
-        -base_wf
-    } else {
-        base_wf
-    };
-    // is_upside_down rotates 180° around the insertion point.
-    let rotation = if resolved_style.is_upside_down {
+    // Backward mirrors text left-right (negative width factor); upside-down
+    // rotates 180° about the anchor. The effective state is the TextStyle flag
+    // XOR the entity's own generation flags (DXF group 71: bit 2 = backward,
+    // bit 4 = upside-down). A MIRROR toggles the entity bit for a true glyph
+    // mirror, and XOR keeps a double mirror an involution.
+    let eff_backward = resolved_style.is_backward ^ (t.generation_flags & 0x2 != 0);
+    let eff_upside = resolved_style.is_upside_down ^ (t.generation_flags & 0x4 != 0);
+    let width_factor = if eff_backward { -base_wf } else { base_wf };
+    let rotation = if eff_upside {
         t.rotation as f32 + std::f32::consts::PI
     } else {
         t.rotation as f32
@@ -111,11 +112,16 @@ fn to_truck(t: &Text, document: &acadrust::CadDocument) -> TruckEntity {
     );
     let (anchor_local_x, anchor_local_y) = if let Some(b) = bounds {
         // Horizontal anchor uses the pen advance box [0, advance] so leading /
-        // trailing spaces keep their width instead of snapping to the inked glyphs.
+        // trailing spaces keep their width instead of snapping to the inked
+        // glyphs. Signed by the width factor: backward text grows in −x, so its
+        // Center/Right offset flips sign too, otherwise the anchor pushes the
+        // box one way while the strokes run the other (the bounds advance is
+        // always positive — it uses |width_factor|). Left keeps its 0 reference.
+        let sign = width_factor.signum();
         let ax = match t.horizontal_alignment {
             HA::Left => 0.0,
-            HA::Center | HA::Middle => b.advance * 0.5,
-            HA::Right | HA::Aligned | HA::Fit => b.advance,
+            HA::Center | HA::Middle => b.advance * 0.5 * sign,
+            HA::Right | HA::Aligned | HA::Fit => b.advance * sign,
         };
         // Vertical anchor uses the inked extent (cap / baseline geometry).
         let ay = match t.vertical_alignment {