Tables: field backgrounds on cells drawn from table data; codec table overrides

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

src/entities/field.rs

@@ -299,6 +299,12 @@ pub fn set_display(on: bool) {
 }
 
 /// 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;
+
 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.