Fields: stored values on redraw, evaluation events, per-field shading

MTEXT draws its stored text instead of re-evaluating fields on every
redraw. Fields update on their evaluation events as the reference does:
open (1), save (2, not autosave), plot (4) and REGEN/REGENALL (16), each
masked with the drawing's FIELDEVAL (variable dictionary, 0-31, with the
reference's prompt and message), and the new UPDATEFIELD command
(selection, attributes of selected block references, "N field(s)
found." / "N field(s) updated."). A Date field (on demand only) keeps
its value until UPDATEFIELD. The FIELDDISPLAY gray box now covers each
field's own glyphs, one box per line it spans, instead of the whole
text. Codec moved to the update_fields / field_spans revision.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/convert/tessellate.rs

@@ -78,6 +78,74 @@ fn oriented_text_corners(
     ]
 }
 
+/// FIELDDISPLAY boxes: the glyph bounds of each field's shown text, one box
+/// per line the field spans, in the text's own frame (`origin`, `rotation`).
+/// Glyph quads come six vertices each in drawing order, one per inked
+/// character, so a field's characters select its quads. `None` when the
+/// host's fields cannot be located in its text (the whole text is boxed).
+fn field_run_corner_groups(
+    verts: &[crate::scene::pipeline::text_gpu::TextVertex],
+    document: &codec::CadDocument,
+    entity: &EntityType,
+    origin: [f64; 2],
+    rotation: f64,
+) -> Option<Vec<[[f64; 2]; 4]>> {
+    use crate::entities::text_support::{mtext_glyph_count, resolve_dxf_special_chars};
+    fn plain_glyph_count(s: &str) -> usize {
+        resolve_dxf_special_chars(s).chars().filter(|c| !c.is_whitespace()).count()
+    }
+    let (text, count): (&str, fn(&str) -> usize) = match entity {
+        EntityType::MText(m) => (&m.value, mtext_glyph_count),
+        EntityType::Text(t) => (&t.value, plain_glyph_count),
+        EntityType::AttributeEntity(a) => (&a.value, plain_glyph_count),
+        _ => return None,
+    };
+    let spans = codec::fields::field_spans(document, entity.common().handle, text)?;
+    let (sin_r, cos_r) = rotation.sin_cos();
+    // Each quad's local bounds [left, bottom, right, top].
+    let quads: Vec<[f64; 4]> = verts
+        .chunks_exact(6)
+        .map(|quad| {
+            let mut b = [f64::MAX, f64::MAX, f64::MIN, f64::MIN];
+            for v in quad {
+                let x = v.pos[0] as f64 + v.pos_low[0] as f64 - origin[0];
+                let y = v.pos[1] as f64 + v.pos_low[1] as f64 - origin[1];
+                let (lx, ly) = (x * cos_r + y * sin_r, -x * sin_r + y * cos_r);
+                b = [b[0].min(lx), b[1].min(ly), b[2].max(lx), b[3].max(ly)];
+            }
+            b
+        })
+        .collect();
+    let to_world = |x: f64, y: f64| [origin[0] + x * cos_r - y * sin_r, origin[1] + x * sin_r + y * cos_r];
+    let mut groups = Vec::new();
+    for span in spans {
+        let first = count(text.get(..span.start)?);
+        let last = count(text.get(..span.end)?).min(quads.len());
+        // A wrapped field starts a new box where the next glyph moves back
+        // left or off the current line.
+        let mut line: Option<[f64; 4]> = None;
+        for q in quads.get(first..last).unwrap_or_default() {
+            line = Some(match line {
+                Some(b) if q[0] >= b[0] && (q[1] + q[3]) * 0.5 > b[1] && (q[1] + q[3]) * 0.5 < b[3] => {
+                    [b[0].min(q[0]), b[1].min(q[1]), b[2].max(q[2]), b[3].max(q[3])]
+                }
+                Some(b) => {
+                    groups.push(b);
+                    *q
+                }
+                None => *q,
+            });
+        }
+        groups.extend(line);
+    }
+    Some(
+        groups
+            .into_iter()
+            .map(|[l, b, r, t]| [to_world(l, b), to_world(r, b), to_world(r, t), to_world(l, t)])
+            .collect(),
+    )
+}
+
 fn oriented_mtext_corner_groups(
     verts: &[crate::scene::pipeline::text_gpu::TextVertex],
     text: &codec::MText,
@@ -1421,12 +1489,29 @@ pub fn tessellate(
                         && crate::entities::field::display()
                         && crate::entities::field::hosts_field(document, entity)
                     {
-                        let corner_groups = match entity {
-                            EntityType::MText(m) => {
-                                let rotation = stroke_groups
+                        let frame = match entity {
+                            EntityType::MText(m) => Some((
+                                [m.insertion_point.x, m.insertion_point.y],
+                                stroke_groups
                                     .iter()
                                     .find_map(|group| group.run.as_ref().map(|run| run.rotation as f64))
-                                    .unwrap_or(m.rotation);
+                                    .unwrap_or(m.rotation),
+                            )),
+                            EntityType::Text(t) => {
+                                Some(([t.insertion_point.x, t.insertion_point.y], t.rotation))
+                            }
+                            EntityType::AttributeEntity(a) => {
+                                Some(([a.insertion_point.x, a.insertion_point.y], a.rotation))
+                            }
+                            _ => None,
+                        };
+                        let runs = frame.and_then(|(origin, rotation)| {
+                            field_run_corner_groups(&sdf_verts, document, entity, origin, rotation)
+                        });
+                        let corner_groups = match entity {
+                            _ if runs.is_some() => runs.unwrap_or_default(),
+                            EntityType::MText(m) => {
+                                let rotation = frame.map_or(m.rotation, |(_, rotation)| rotation);
                                 oriented_mtext_corner_groups(&sdf_verts, m, rotation, 0.0, anno)
                             }
                             EntityType::Text(t) => vec![oriented_text_corners(