iPhone app: test pairing and power against a Holo Core stand-in

Valve publishes no Steam Frame OS image, so tests/frame-container builds the
Frame's SSH surface on Valve and Collabora's Holo Core aarch64 base: a
steamos user with a password and sudo, OpenSSH with keys and passwords,
Python, and a systemctl that only records requests.

Against it from the Simulator: password pairing, the host-key pin, the power
password check. Found and fixed: a changed host key or a refused login said
"Can't reach the Frame" and retried forever; they now say "Pair with the
Frame again" and offer that. The server's key rejection now says the header
may be wrong, not only missing.

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

tests/frame-container/README.md

@@ -0,0 +1,20 @@
+# A Frame stand-in for testing without the headset
+
+Valve publishes no Steam Frame OS image. This builds the closest thing: Valve and
+Collabora's [Holo Core aarch64 preview](https://www.collabora.com/news-and-blog/news-and-events/building-an-arch-linux-aarch64-port-for-holo-core.html)
+(the Arch Linux ARM64 base the Frame's SteamOS is built on) with the Frame's SSH
+surface: a `steamos` user with a password and sudo, OpenSSH taking keys and
+passwords, Python and rsync. `systemctl` only records what it's asked to do.
+
+It exercises pairing with the password, the host-key pin, the server running on
+the "Frame" (FRAME_LOCAL=1) and the power password check. It has no SteamVR,
+Steam, battery, cameras or Lepton, so those panels are empty.
+
+```sh
+docker build --platform linux/arm64 -t frame-holo-test tests/frame-container
+docker run -d --name frame-holo -p 127.0.0.1:2222:22 frame-holo-test
+# password: frame-test-pw. In the iPhone app (Simulator), pair with 127.0.0.1:2222.
+docker exec frame-holo cat /tmp/power-requests.log   # what power actions asked for
+```
+
+On a Mac without Docker: `brew install colima && colima start --arch aarch64 --vm-type vz`.