review(#1630): one table rebuild loop that also refreshes formulas; one cell layout

- main already rebuilt an edited table's block; the PR added a second
  loop after it that could never fire. Formula refresh and the block
  cache bump now live in the one loop.
- Formula refresh copies the drawing only for a table that has a field.
- A field cell lays its text out once and boxes it from those cells.
- A zero text height at one level falls through to the next instead of
  ending the lookup.
- Field box constants and BACKGROUND keep their own doc comments.

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

src/entities/field.rs

@@ -137,7 +137,17 @@ pub fn update_fields(
 /// Recompute a table's cell formulas into its block after the table changed;
 /// returns the cell texts that changed.
 pub fn refresh_table_formulas(document: &mut CadDocument, table: Handle) -> Vec<Handle> {
-    if document.fields.is_empty() {
+    // Copying the drawing is only worth it for a table with a field.
+    let has_field = matches!(
+        document.get_entity(table),
+        Some(codec::EntityType::Table(t)) if t
+            .rows
+            .iter()
+            .flat_map(|row| &row.cells)
+            .flat_map(|cell| &cell.contents)
+            .any(|content| content.field_handle.is_some())
+    );
+    if !has_field {
         return Vec::new();
     }
     let snapshot = document.clone();
@@ -332,13 +342,13 @@ pub fn set_display(on: bool) {
     FIELD_DISPLAY.store(on, std::sync::atomic::Ordering::Relaxed);
 }
 
-/// The gray the reference draws behind a field.
 /// FIELDDISPLAY box extent around a line of field text, in text heights:
 /// from a third of the height below the baseline to 1.19 heights above it
 /// (the reference's box, measured at 2.5 and 10 unit text).
 pub const BOX_BELOW: f64 = 0.345;
 pub const BOX_ABOVE: f64 = 1.19;
 
+/// The gray the reference draws behind a field.
 pub const BACKGROUND: [f32; 4] = [203.0 / 255.0, 203.0 / 255.0, 203.0 / 255.0, 1.0];
 
 /// Whether `entity` hosts a field: its extension dictionary has ACAD_FIELD.