docs: record what the passing runs established

Both variants now pass, and the with-prerequisites run settled several things
that were guesses before:

* The Apple `container` service is per-user, confirmed directly rather than
  inferred: the throwaway account's appRoot is its own
  (/Users/bbtest/Library/Application Support/com.apple.container/), and
  starting it left the admin's service running and its agents intact.
* Setting up a new account is two steps, not one. The guest kernel lives in
  that same per-user app root, so a fresh account has none and
  `container system start` prompts to download it — and only prompts, since
  the flags default to asking. Headless callers need
  --enable-kernel-install.
* Neither step is performed by `bot-bottle backend setup
  --backend=macos-container`, which checks and then defers to `container
  system start`. So the real path for a new account is
  `container system start --enable-kernel-install`.
* Any of it requires entering the user's launchd domain with `launchctl
  asuser`, because the apiserver is a per-user agent reached over XPC.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WEfZZhakx13bxTfXcZCoS5

docs/research/testing-clean-install-on-macos.md

@@ -237,10 +237,35 @@ service is per-user. It asserts the install is sound and *reports* backend
 readiness without failing on it, because install.sh provides no backend and so
 cannot regress one.
 
-`test-ready` models a system **with** them: it runs `container system start`
+`test-ready` models a system **with** them, then demands doctor go fully green,
-for the throwaway user, then demands doctor go fully green, backend included.
+backend included. Both variants pass as of this writing.
-(`bot-bottle backend setup --backend=macos-container` does not start the
+
-service — it only checks and tells you to run that command yourself.)
+### The per-user prerequisite is two steps, not one
+
+Running it revealed that "set up the backend for this account" is more than
+starting a service:
+
+1. **`container system start`** — the service is per-user. The run confirms it
+   directly: the throwaway account's `appRoot` is
+   `/Users/bbtest/Library/Application Support/com.apple.container/`, its own,
+   and starting it left the admin's service untouched.
+2. **A guest kernel**, which also lives in that per-user app root. A fresh
+   account has none, so `container system start` prompts to download one —
+   and *only* prompts, since the flags default to asking. Headless callers
+   must pass `--enable-kernel-install` or the command dies on
+   `failed to read user input`.
+
+Neither step is done by `bot-bottle backend setup --backend=macos-container`,
+which only checks and then tells you to run `container system start` yourself.
+So a new account's real path to a working backend is:
+`container system start --enable-kernel-install`.
+
+The harness must also enter the user's launchd domain via
+`launchctl asuser <uid>` to do any of this. `container system start` registers
+`com.apple.container.apiserver` as a per-user launchd agent and talks to it
+over XPC; from plain `sudo -u` the caller is still in root's bootstrap
+namespace, the lookup crosses domains, and the apiserver answers
+`invalidState: "unauthorized request"` even though the agent started fine.
 
 It refuses to start against an existing account (a reused home is not a clean
 install), and it tears the account down from an `EXIT`/`INT` trap armed the