Add Steam Deck button prompts beside the Xbox and PlayStation ones (prompts = steamdeck).

The Controls tab's Button prompts row and nx2navi.ini take steamdeck: the Deck's own labels in the game's
text, action prompt and ocarina notes, which are A B X Y where an Xbox pad has them, L1 R1 L2 R2 and L3 R3,
Menu and View. The style is drawn by tools/pad_prompts.py like the others, into romfs-patches-steamdeck
and romfs-patches-steamdeck-label, and every build with button art writes them.

Pull request #107 by loteran asked for PlayStation and Steam Deck prompts; it was closed, and the tree
took the PlayStation half in another form, drawn from button art instead of redrawn font glyphs. This is
the Steam Deck half in that form, written here: the art has no Deck style, so its entry in
tools/prompt_art.py names the Xbox pictures for the face buttons, Menu, View, the d-pad and the sticks
and the PlayStation pictures for the shoulders and stick clicks. It needs four pictures of the Xbox set
that no other style used (the white letters on a light button, for the ocarina's notes); a release made
with --prompt-art copies them with the rest.

The Windows files compile and tests/launcher_test.py passes, with the prompts step's test following the
number of folders (seven now); every picture the style names is in the art used for releases. The style
was not drawn here (that needs the game's font), and --probe-cfg, which has a new check, was not run.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/README-BuildKit.md

@@ -25,7 +25,7 @@ downloads included, and keeps the game up to date. This page is the same build b
    ```
 
    `zbic` unpacks the game's compressed program files. It is a separate GPL-2.0 package you install yourself; the kit
-   only calls it. `fonttools` and `pillow` draw the Xbox, PlayStation and keyboard and mouse button prompts, when the
+   only calls it. `fonttools` and `pillow` draw the Xbox, PlayStation, Steam Deck and keyboard and mouse button prompts, when the
    kit has a `prompt-art` folder.
 
 2. **MSYS2** from [msys2.org](https://www.msys2.org/), installed to the default `C:\msys64`. Open **MSYS2 UCRT64** from
@@ -53,7 +53,7 @@ The build checks the version, unpacks the program files, packs the 26 GB RomFS i
 the dump's own checksums) while it turns the game's code into C and compiles it (about 6 minutes on a 16-thread
 Ryzen 7; longer on fewer cores), then prepares every one of the game's shaders for your graphics card (about 5 more
 minutes), so the game does not stop to compile shaders while you play. When the kit has a `prompt-art` folder, it
-last draws the Xbox, PlayStation and keyboard and mouse button prompts from that art (about a minute) into the
+last draws the Xbox, PlayStation, Steam Deck and keyboard and mouse button prompts from that art (about a minute) into the
 `romfs-patches-*` folders beside the game. The game it builds is for this PC's processor. You can use your PC
 meanwhile. If it stops, run the same command again: finished steps are skipped.
 
@@ -150,7 +150,7 @@ beside the game.
   upscaling on, and the frame limit then counts the frames on screen; with no limit chosen it is the display's refresh
   rate), HDR (the game's picture tone mapped to your display, with its own grading rows, or Native, the game's own HDR; turn HDR on in Windows first) and the
   resolution. Those marked "after restart" take effect when the game starts again.
-- **Controls** in the menu has Button prompts: the game's own, or Xbox, PlayStation or keyboard and mouse ones when
+- **Controls** in the menu has Button prompts: the game's own, or Xbox, PlayStation, Steam Deck or keyboard and mouse ones when
   the build drew them (a kit with `prompt-art`). It takes effect after a restart. The keyboard and mouse prompts show
   the keys `nx2navi.ini` binds when the build runs: after rebinding keys, run
   `py -I tools\make_game.py --rebuild --out "C:\Games\Ocarina"` to draw them again (it skips everything else).