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>

frame/devkit-utils/steam-devkit-rpc

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+
+import sys
+import os
+import logging
+import argparse
+import tempfile
+import urllib.parse
+import re
+
+import devkit_utils
+
+logging.basicConfig(format='%(message)s', level=logging.DEBUG)
+logger = logging.getLogger()
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+    parser.add_argument('command')
+    parser.add_argument('args', nargs='*')
+    conf = parser.parse_args()
+
+    try:
+        devkit_utils.validate_steam_client()
+    except devkit_utils.SteamClientNotRunningException as e:
+        logger.error(repr(e))
+        sys.exit(-1)
+    else:
+        with tempfile.TemporaryDirectory(prefix='steam-devkit-rpc') as tempdir:
+            response = os.path.join(tempdir, 'steam-devkit-rpc')
+            parms = {
+                'response' : response,
+            }
+            for arg in conf.args:
+                (k, v) = re.split('=', arg)
+                parms[k] = v
+            cmd = f'{conf.command}/?{urllib.parse.urlencode(parms)}'
+            devkit_utils.execute_steam_client_command(cmd)
+            try:
+                with devkit_utils.wait_on_file_response(response) as success_response:
+                    logger.info('success')
+                    sys.stdout.write(success_response)
+                    sys.exit(0)
+            except devkit_utils.SteamResponse_Timeout:
+                logger.error('timeout')
+            except devkit_utils.SteamResponse_Error as e:
+                logger.error('failed')
+                sys.stdout.write(e.error_response)
+                sys.exit(-1)