Variables: a text value on threaded type replaces the whole story

Applying a data set wrote a text value into the bound frame only, and the
thread's reflow then joined it with the other frames' old text. The art no
longer matched the row it had just applied, a captured row applied back lost
text, and a value bound to a later frame deleted the rest of the story.

- A text value bound to any frame of threaded text replaces the whole story,
  which then flows through the frames again. `overflow` names the story's
  last frame.
- Capture Data Set, Update Data Set and the Variables panel's match check
  read such a frame as the whole story, frames joined in thread order.
- When any frame of the story is locked, the text value is skipped and
  counted in `skipped`, and the story is not read.
- Two text variables bound in one story apply in definition order, as for
  two on one object: the one defined last wins.

Docs: docs/mcp.md.

crates/engine/src/cmd/variablecmds.rs

@@ -18,7 +18,7 @@ use vectorcraft_doc::{DataSet, DataValue, NodeId, NodeKind, Variable, VariableKi
 
 use super::edit::selected_roots;
 use super::fileio::{absolute_path, read_file_head, write_file};
-use super::textedit::set_plain_text;
+use super::textedit::set_story_text;
 use super::{CommandSpec, EngineError, Result, Session, bad, bool_or, checked_ids_or_selection, cmd, has_doc, has_selection, links, str_param};
 
 mod csv;
@@ -487,16 +487,20 @@ fn apply_dataset(s: &mut Session, name: &str) -> Result<Value> {
             };
             // Locked art, art in a locked layer or group, and text whose object is gone, can't be
             // rewritten: skipped and counted, like the rest. Hidden art is rewritten, in a hidden
-            // layer or group too, and stays hidden unless a visibility value shows it.
-            if !d.dataset_target(id) {
+            // layer or group too, and stays hidden unless a visibility value shows it. A text value
+            // rewrites the whole story of threaded text, so it skips when any frame of it is locked.
+            let target = if kind == VariableKind::Text { d.story_target(id) } else { d.dataset_target(id) };
+            if !target {
                 skipped += 1;
                 continue;
             }
             let done = match (kind, &value) {
                 // The Type tool's own whole-range replace, so a dataset's text is styled and
-                // laid out exactly as typing it would be.
-                (VariableKind::Text, DataValue::Text(text)) => set_plain_text(d, id, text).is_ok_and(|()| {
-                    typed.push(id);
+                // laid out exactly as typing it would be. A frame of threaded text has its whole
+                // story replaced, the value capture reads from it; the story's last frame is the
+                // one that overflows.
+                (VariableKind::Text, DataValue::Text(text)) => set_story_text(d, id, text).is_ok_and(|last| {
+                    typed.push(last);
                     true
                 }),
                 (VariableKind::Visibility, DataValue::Visible(show)) => d.node_mut(id).is_some_and(|n| {
@@ -670,7 +674,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Capture Data Set",
             ["Window", "Variables"],
             None,
-            "{name?} record what the bound objects hold right now as a new dataset (Data Set 1…), and make it the active one",
+            "{name?} record what the bound objects hold right now as a new dataset (Data Set 1…), and make it the active one; a frame of threaded text holds its whole story, every frame's text in thread order",
             has_doc,
             dataset_capture
         ),
@@ -706,7 +710,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Select Data Set",
             ["Window", "Variables"],
             None,
-            "{name} apply a dataset in one undo step, deselecting every selected object left hidden or locked (the key object too) → {applied, skipped, overflow?: [id…] (area type and type on a path whose text it rewrote and that no longer fits; left out when all fits), warning? (when no object is bound to a variable that the dataset has a value for, so nothing was applied)}",
+            "{name} apply a dataset in one undo step, deselecting every selected object left hidden or locked (the key object too); a text value bound to a frame of threaded text replaces the whole story → {applied, skipped, overflow?: [id…] (area type and type on a path whose text it rewrote and that no longer fits, the story's last frame for threaded text; left out when all fits), warning? (when no object is bound to a variable that the dataset has a value for, so nothing was applied)}",
             has_doc,
             dataset_select
         ),