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/scene/modify.rs

@@ -32,13 +32,24 @@ fn capture_text_orient(e: &EntityType) -> Option<TextOrient> {
     }
 }
 
-/// Re-apply a captured orientation after a transform (the position stays
-/// mirrored, only the rotation / oblique / x-scale are restored).
+/// MIRRTEXT off: after the mirror reflects a text's points and rotation, put
+/// the glyphs back to right-reading. Restores the captured rotation / oblique /
+/// x-scale, and for single-line TEXT also flips the horizontal justification
+/// (left ↔ right) so the box lands mirror-symmetric to the source instead of
+/// hugging the axis — AutoCAD's behaviour. Center/Middle/Aligned/Fit are already
+/// symmetric about their (reflected) anchor, so they stay.
 fn restore_text_orient(e: &mut EntityType, o: &TextOrient) {
     match e {
         EntityType::Text(t) => {
             t.rotation = o.rotation;
             t.oblique_angle = o.oblique;
+            use acadrust::entities::TextHorizontalAlignment as HA;
+            t.horizontal_alignment = match t.horizontal_alignment {
+                HA::Left => HA::Right,
+                HA::Right => HA::Left,
+                other => other,
+            };
+            crate::entities::text::sync_text_alignment_point(t);
         }
         EntityType::MText(m) => {
             m.rotation = o.rotation;
@@ -52,6 +63,19 @@ fn restore_text_orient(e: &mut EntityType, o: &TextOrient) {
     }
 }
 
+/// MIRRTEXT on: a real MIRROR makes text a true glyph mirror. `apply_transform`
+/// already reflects the baseline rotation; toggling *both* group-71 bits
+/// (backward `0x2` + upside-down `0x4`) then renders the exact geometric
+/// reflection (the renderer XORs these flags into the effective width-factor
+/// sign / 180° flip). Toggling — not setting — keeps a double mirror an
+/// involution. Only single-line TEXT carries group 71; MTEXT / Shape keep the
+/// reflected rotation (readable-but-rotated), which is standard-conforming.
+fn mirror_true_text_flags(e: &mut EntityType) {
+    if let EntityType::Text(t) = e {
+        t.generation_flags ^= 0x2 | 0x4;
+    }
+}
+
 impl Scene {
     // ── Modify (transform / copy) ─────────────────────────────────────────
 
@@ -67,6 +91,10 @@ impl Scene {
         // transform and re-apply afterwards.
         let preserve_text_orientation =
             matches!(t, EntityTransform::Mirror { .. }) && !self.document.header.mirror_text;
+        // MIRRTEXT on: toggle the group-71 flags after the reflect so text
+        // becomes a true glyph mirror (see `mirror_true_text_flags`).
+        let mirror_true =
+            matches!(t, EntityTransform::Mirror { .. }) && self.document.header.mirror_text;
         let mut text_orient_backup: Vec<(Handle, TextOrient)> = Vec::new();
         if preserve_text_orientation {
             for &h in handles {
@@ -106,6 +134,9 @@ impl Scene {
         for &h in handles {
             if let Some(entity) = self.document.get_entity_mut(h) {
                 view::dispatch::apply_transform(entity, t);
+                if mirror_true {
+                    mirror_true_text_flags(entity);
+                }
             }
             if self.hatches.contains_key(&h) {
                 let existing_color = self.hatches[&h].color;
@@ -245,6 +276,8 @@ impl Scene {
         // header flag is off.
         let preserve_text_orientation =
             matches!(t, EntityTransform::Mirror { .. }) && !self.document.header.mirror_text;
+        let mirror_true =
+            matches!(t, EntityTransform::Mirror { .. }) && self.document.header.mirror_text;
         let mut new_handles = Vec::with_capacity(clones.len());
         for mut entity in clones {
             let text_orient = if preserve_text_orientation {
@@ -256,6 +289,9 @@ impl Scene {
             if let Some(o) = &text_orient {
                 restore_text_orient(&mut entity, o);
             }
+            if mirror_true {
+                mirror_true_text_flags(&mut entity);
+            }
             // A dimension draws from its baked `*D` block; give the copy its own
             // transformed block so it lands at the copy position rather than
             // rendering on top of the source. (#161)