Variables: Import Data Sets… defines the variables a file's header names

dataset.import refused every header cell that wasn't the name of a
variable the document defines. A data source file could only fill
variables made beforehand, and `#` and `@` in a header were read as part
of a variable's name.

A header cell that names no variable now defines a new, unbound
variable in the import's undo step. `#Shown` defines a visibility
variable `Shown`, `@Photo` defines an image variable `Photo`, and a
cell without a prefix defines a text variable. A prefixed cell whose
name is already a variable of the prefix's kind names it. A cell that
is a variable's exact name keeps naming it, even when the name starts
with `#`, `@` or `%`, so files that dataset.export writes import back
as before. The import fails, naming the column, on a prefixed cell
whose name is a variable of another kind and on a `%` cell that names
no variable (graph data files). While the variables are locked, a
header that would define a variable fails the import with "variables
are locked", as New Variable does. The reply's `defined` lists the new
variables.

Docs: docs/mcp.md, docs/target-app-parity.md.

docs/mcp.md

@@ -1130,15 +1130,20 @@ already walks Image Trace. Short agent entry points:
   values CSV as `graph.setData`). `variable.lock {locked?}` (Lock Variables) locks or unlocks the variables;
   without `locked` it toggles, and the lock is saved with the document. While the variables are locked,
   `variable.define`, `variable.delete`, `variable.rename` and the four `variable.make…Dynamic` commands fail with
-  "variables are locked"; `variable.bind`, `variable.unbind` and the `dataset.*` commands still run, and
-  `variable.list` reports `locked`.
+  "variables are locked"; `variable.bind`, `variable.unbind` and the `dataset.*` commands still run (except a
+  `dataset.import` whose header would define a variable, below), and `variable.list` reports `locked`.
 - Data sets: `dataset.list`, `dataset.new {name, values?}`, `dataset.set {name, values?}`, `dataset.rename`,
   `dataset.delete`, `dataset.capture`, `dataset.update`, `dataset.select {name}`, `dataset.next` /
   `dataset.prev` (each `dataset.select` / next / prev is one undo step over every bound object), `dataset.import {path|data|dataBase64|csv}` /
   `dataset.export {path?}` (CSV: `name`, then one column per variable). `dataset.import` also reads tab-separated
   files, and files whose first header cell isn't `name`: every header cell is then a variable, and the 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.
+  line the row starts on. A header cell that names no variable defines one, unbound, in the same undo step as the
+  rows. `#Shown` defines a visibility variable `Shown`, `@Photo` defines an image variable `Photo`, and a cell without
+  a prefix defines a text variable. A prefixed cell whose name is already a variable of the prefix's kind names it;
+  one whose name is a variable of another kind fails the import. A `%` cell that isn't a variable's exact name (graph
+  data files) is refused. The reply's `defined` lists the variables the import defined. While the variables are
+  locked, an import whose header would define a variable fails with "variables are locked".
 - 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

@@ -433,6 +433,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - Export for Screens, Save for Web (slice images, and files named after the document's title) and the Libraries panel name files with the characters Windows does not allow written as `-` and with a `-` after a Windows device name such as `CON`, on every platform (`document.exportForScreens`, `document.exportForWeb`, `library.create`).
   - Variables (data merge): text, visibility and image variables bound to objects (an object may carry several), dataset rows applied in one undo step (`variable.*`, `dataset.*`) from a Variables panel and Window › Variables, with dialogs to define, bind and edit, Make Text/Visibility/Image Dynamic, Capture/Update Data Set, CSV import/export (`dataset.import` / `dataset.export`), and a row in italics when an edit leaves the art not matching it; an image dataset value is a file path and apply keeps the object's bounds; bindings let go of deleted art and locked art is left alone; every command, like the rest, is drivable over MCP and the CLI.
   - 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.
   - 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.