Look the host list up by name, rather than off a field that is never assigned

Nightly 0.0.0-alpha.0.133 died before its first frame. The long press I added
attached itself in HostsScreen's constructor through the field the Avalonia name
generator declares for `x:Name` — and that field is assigned by the generated
InitializeComponent, which no view in this repository calls. Every one of them
loads its XAML directly. So the field compiles, resolves in the editor, and is
null at run time; PhoneShell builds this control on the way up, so the
NullReferenceException took the launch rather than the hosts screen.

PhoneShell and TerminalScreen both use FindControl, and PhoneShell carries a
<remarks> saying exactly this and naming exactly this consequence. I read neither
and wrote the field.

So the rule is in docs/platform-flags.md now as well. A comment on the control
that already got it right is not where somebody writing a new one is looking,
which is the whole of why two correct examples and one warning were not enough.

And phase 8 opens with "it launches at all". Nothing on this head is covered by a
test — no test project, no headless surface — so a view that throws while being
built takes the launch with it and no gate anywhere says so. Thirty seconds, and
it would have caught this one before it was published.

docs/manual-checks.md

@@ -904,6 +904,19 @@ the rectangles are not. So what follows is the rectangles.
 
 Run on a 360dp-wide device or emulator, unlocked, on HOSTS.
 
+### 8.0 It launches at all · **thirty seconds, and it has caught a shipped crash**
+
+Install the APK and open it.
+
+**Pass:** the opening screen appears.
+
+**Failure means:** nothing on this head is covered by a test — see the paragraph above — so a view that
+throws while it is being built takes the launch with it and no build gate anywhere says so. The one that
+shipped was a generated `x:Name` field used from a constructor, which is null in every view in this
+repository; see `docs/platform-flags.md`. Get the stack with `adb logcat -d -b crash`, which names the view.
+
+Every check below assumes this one passed, and none of them can be reached if it did not.
+
 ### 8.1 The + is reachable and is not covering anything
 
 Look at the bottom right of the host list with no host selected.