Variables: Update Data Set writes only the variables the active data set holds

A data set can hold values for only some variables: `dataset.new` and
`dataset.set` store only the values they are given, and selecting the
data set leaves the other variables alone. Update Data Set replaced the
active data set's values with the values the art reports for every
variable. After one Update, a data set that set only a visibility
variable also held the current text of the bound type, and selecting it
again wrote that text back. Update also dropped a variable's value when
its objects were locked or inside a hidden or locked layer or group,
because such objects report no value.

`dataset.update` now writes the art's current value for each variable
the active data set holds, and keeps the data set's value when the art
reports none. A variable that the data set does not hold stays out of
it, including one defined after the data set was captured;
`dataset.update {all: true}` also adds the other variables that the art
reports a value for. The reply adds `left`, the variables it did not
write. The Variables panel's Update Data Set button and menu item run
it without `all`.

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

docs/mcp.md

@@ -972,6 +972,12 @@ already walks Image Trace. Short agent entry points:
 {"name":"run_command","arguments":{"command":"imageTrace.presets","params":{}}}
 ```
 
+`dataset.update {all?}` writes what the bound objects hold now into the active data set, for each variable that the
+data set holds. A variable whose objects report no value, because they are locked, inside a hidden or locked layer
+or group, or deleted, keeps the data set's value. With `all: true` it also adds every other variable that the
+objects report a value for. The reply gives `values`, the number of values it wrote, and `left`, the variables it
+did not write.
+
 ## Graphic styles
 
 A graphic style holds an appearance (fills, strokes, effects) plus opacity, blend mode, isolate and knockout.

docs/target-app-parity.md

@@ -396,6 +396,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 
 - **Architecture:** 19+ crates with enforced layering (`cargo xtask layers`). Every action is a command (~400 engine + ~50 UI). Undo is unlimited via structural sharing. `command.batch` runs several commands as one transaction.
 - **Automation:**
+  - Variables: Update Data Set writes what the art holds into the active data set for the variables that data set holds, and keeps the data set's value for a variable whose objects are locked or inside a hidden or locked layer or group; `dataset.update {all: true}` also adds the other variables that the art reports a value for.
   - Actions panel (record/playback, persisted), generic parameter dialogs for every "…" command.
   - Variables (data merge): text and visibility 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 Dynamic, Capture/Update Data Set, and a row in italics when an edit leaves the art not matching it; bindings let go of deleted art and locked art is left alone; every command, like the rest, is drivable over MCP and the CLI.
   - JSON-lines control channel with real egui pointer and keyboard injection.