Variables: Save Variable Library… writes the variables and data sets as an XML library

Squashed from #5147 onto main.

docs/mcp.md

@@ -1679,6 +1679,18 @@ already walks Image Trace. Short agent entry points:
   comments, processing instructions and up to 64 entity declarations whose values are plain text (no `<`, `&` or
   `%`). A declaration that holds anything else, and entity references that expand the library past 8 MiB, are
   refused. So are a variable or data set declared twice and a value for a variable the library doesn't declare.
+  `variable.saveLibrary {path?}` (the panel menu's Save Variable Library…) writes the document's text variables and
+  every data set as a variable library in the layout `variable.loadLibrary` reads: a `variable` element with
+  `varName` and `trait="textcontent"` for each text variable, and a `sampleDataSet` element with `dataSetName` for
+  each data set, holding an element named after each text variable it has a value for, with one `p` element per line
+  of the value. Loading the file gives back the same text variables, data set names and text values. Variables of
+  other kinds are left out with their values: the reply lists them in `skipped` (`name`, `kind`), and its `warning`
+  names each kind with its count. Saving is refused for a document without a text variable, for a text variable whose
+  name isn't an XML name (it starts with a letter or `_` and holds only letters, digits, `-`, `_` and `.`), for a
+  data set whose name or text value holds a control character, for a data set name with white space at either end,
+  for a name longer than 255 characters or used twice, and past `variable.loadLibrary`'s caps (256 variables, 1,000
+  data sets, 8 MiB). Without `path` the reply is `{xml, variables, datasets, skipped, warning?}`;
+  with it, the file is written and the reply is `{path, variables, datasets, skipped, warning?}`.
 - Batch: `action.batch {commands, source?: folder|datasets, folder?, destination?: none|saveAndClose|folder,
   destinationFolder?, overrideOpen?, overrideSave?, overrideExport?, includeSubdirs?, stopOnError?}` plays an
   action on every file in a folder or every data set (the Actions panel menu's Batch…). Override Save and Override

docs/target-app-parity.md

@@ -565,6 +565,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - Variables: Import Data Sets… (`dataset.import`) reads tab-separated files as well as comma-separated ones, and files without a `name` column, whose rows take the first free names Data Set 1, Data Set 2 and so on. An import error about a row's values or name gives the file line the row starts on.
   - Variables: Import Data Sets… (`dataset.import`) defines the variables a file's header names that the document lacks, unbound. `#Shown` defines a visibility variable, `@Photo` defines an image variable, and a header without a prefix defines a text variable. A header that is a variable's exact name keeps naming it, and any other `%` header (graph data files) is refused.
   - Variables: Load Variable Library… in the Variables panel menu (`variable.loadLibrary`) replaces the document's variables and data sets with a variable library's, read from its XML file, in one undo step: every variable is deleted with its bindings and values, and every data set is deleted; then the library's text variables are defined, unbound, and its data sets take the text of each value's `p` elements, one paragraph each. Variables of other kinds are left out with a warning that names their kinds.
+  - Variables: Save Variable Library… in the Variables panel menu (`variable.saveLibrary`) writes the document's text variables and data sets as a variable library's XML file, which Load Variable Library reads back with the same text variables, data set names and text values. Variables of other kinds are left out with a warning that names their kinds.
   - JSON-lines control channel with real egui pointer and keyboard injection.
   - Synthetic input validates complete gestures before queue mutation: at most 256 drag steps, 16 clicks and 1,024 pending events; up to 64 control requests per frame, with later requests retained for the next frame (#1709).
   - The control server writes each reply through a 64 KiB buffer. On Windows 11 it sends the 17 MB `document.inspect` reply of a document with 50,000 objects on one layer to a client over loopback in 0.09 s instead of 34 s.