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 <[email protected]>

src/entities/text_support.rs

@@ -488,6 +488,22 @@ fn font_stem(name: &str) -> String {
         .to_string()
 }
 
+/// How many inked glyphs (non-blank characters, stacked parts included) an
+/// MTEXT string draws, in drawing order: the index of the first glyph of
+/// what follows `s` in a longer string.
+pub fn mtext_glyph_count(s: &str) -> usize {
+    let ink = |t: &str| t.chars().filter(|c| !c.is_whitespace()).count();
+    adapt_mtext_paragraphs(s, 1.0, false)
+        .iter()
+        .flat_map(|line| &line.runs)
+        .map(|run| match &run.kind {
+            MTextRunKind::Glyphs(text) => ink(text),
+            MTextRunKind::Tab => 0,
+            MTextRunKind::Stack { numerator, denominator, .. } => ink(numerator) + ink(denominator),
+        })
+        .sum()
+}
+
 /// Parse an MTEXT string into the layout's `Vec<MTextLine>`, using opencadcodec's
 /// structured `mtext_format::parse_mtext` — OCS keeps only the layout engine
 /// (`layout_mtext` and callers read `MTextLine`/`RunState`), not a second MTEXT