Sideload Linux and Windows builds as Steam Devkit Games

Dropping a game's .zip, folder or .exe on Send to Frame now adds it to the
headset's Steam library through Valve's SteamOS Devkit title path, with the
runtime picked from the program's header: Windows PE -> Proton Experimental
(steam_play=1), aarch64 ELF -> SteamLinuxRuntime_4-arm64, x86-64 ELF ->
SteamLinuxRuntime_4 (through FEX). Other architectures are refused.

- frame/devkit-utils: Valve's devkit-utils vendored unmodified (MIT,
  steamos-devkit v0.20260925.1), synced to ~/devkit-utils by stamp, bundled in
  the app and compiled in CI.
- ui/frame_titles.py: inspect (safe unzip, ELF/PE classification, launch
  target ranking), install(path, name=None, exe=None, runtime=None,
  progress=None), list, launch, remove, plus a CLI.
- ui/server.py: /api/titles (inspect/install/discard/launch/remove),
  /api/titles/job progress, and an upload mode 'title'.
- ui/index.html: confirm dialog (name, launch target, runtime), install
  progress, and a Sideloaded titles list with Launch and Remove. The app's
  preload passes a dropped folder's path.
- tests and docs/sideloading.md. Device-side behaviour is inferred from
  Valve's source; the headset was offline, so none of it has been checked on
  a Frame yet.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/frame-control.md

@@ -56,7 +56,10 @@ python3 ui/server.py         # anywhere: then open http://127.0.0.1:47810
   (0.85–1.3×) over ADB (`wm size`, `wm density`, `font_scale`). Reset puts all
   three back. Whether the settings survive the app relaunching is untested.
 - **Transfer**: drag and drop files to `~/Downloads`; `.apk` files install as
-  their own Android app. Send typed text, or your computer's clipboard, to the
+  their own Android app. A game's `.zip`, folder or `.exe` becomes a title in
+  the Steam library (Valve's Devkit Game path, with Proton or the Steam Linux
+  Runtime picked from the program's header), listed under **Sideloaded titles**
+  with Launch and Remove; see [sideloading.md](sideloading.md). Send typed text, or your computer's clipboard, to the
   Frame clipboard.
 - **Flatpaks**: install and remove them (quick picks: Moonlight, Firefox, VLC,
   Remmina).
@@ -69,7 +72,7 @@ python3 ui/server.py         # anywhere: then open http://127.0.0.1:47810
 
 `app/` is an Electron shell. It starts `ui/server.py` on a free loopback port
 and shows it in its own window; the server stops when you quit the app. The
-app bundles `ui/`, `scripts/`, `frame/android/` and the rated catalogue from
+app bundles `ui/`, `scripts/`, `frame/android/`, Valve's `frame/devkit-utils/` and the rated catalogue from
 `apk-catalog/`, plus a standalone Python
 ([python-build-standalone](https://github.com/astral-sh/python-build-standalone))
 and `adb` from Google's platform-tools, so there's nothing else to install.
@@ -93,7 +96,7 @@ library capsules and green Play buttons.
 both capture modes (headset view while in use, and a blank frame in standby,
 which the UI labels), clipboard, volume, file push, and input validation.
 **Not yet exercised from the UI:** Launch, Flatpak install/remove, APK drop,
-and the power buttons. Each of these calls a command that was verified
+title sideloading (not yet run on a headset at all), and the power buttons. Each of these calls a command that was verified
 separately.
 
 ## Per-platform notes