Export for Screens: the dialog's example names the file the export writes

The Export for Screens dialog built its Example line from its fields as
typed, so it often showed a name the export does not write. It used the
artboard's name with its spaces (Artboard 1.png where the export writes
Artboard-1.png) and the first artboard whatever the range, kept a
prefix, a suffix or a device name such as CON as typed, used the scale
text as the row folder instead of the row's own folder or its size,
kept a vector row's @2x suffix, left out the folder that Create
Sub-folders adds, and read a counter set as text as 1.

The line now comes from the export's own naming. The naming part of
document.exportForScreens moves into a Naming type that the export
uses for every file, and fileio::screen_first_file returns the path of
the first file the export writes for the given params, without encoding
or writing anything. The dialog passes it the params it exports with,
without the destination, so the line shows before a folder is chosen
and is left out while the settings choose nothing to export. The Start
at field shows the counter the export uses when it was set as text.

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

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
   - Batch…: Override Save and Override Export name each file with the extension of the format the step writes, however the step spells the format (`.svg` and `SVG` give `.svg`, `png8` gives `.png`), in `action.batch`.
   - Actions panel (record/playback, persisted), generic parameter dialogs for every "…" command.
   - Batch… over data sets names each file after the document and the data set (`action.batch`). It writes `/`, `\`, `:`, `<`, `>`, `"`, `|`, `?`, `*` and control characters in the data set's name as `-`, and adds `-2`, `-3`… to a name an earlier data set used.
+  - Export for Screens: the dialog's Example line shows the path of the file the export writes first, sub-folder included, named as the export names it, for the first chosen artboard, asset, layer or object (`document.exportForScreens`).
   - 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.