Stop the terminal's accessory keys taking the keyboard off it

Ctrl, Esc, Tab, the arrows and the two text-size keys were ordinary Avalonia
buttons sitting over a NativeWebView. An ordinary button takes focus on tap,
which takes it off the WebView — and the package's own OnLostFocus then calls the
adapter's ResignFocus(). So pressing Tab handed the terminal one byte and took
the keyboard away from it, and everything typed afterwards went nowhere.

What makes it worth more than a one-line fix is the symptom. The row goes on
working, because its keys are pressed rather than typed into, so what you see is
a terminal that answers the buttons and ignores the keyboard — which reads as the
session having died rather than as anything to do with focus.

Focusable = false is what a toolbar button is: these keys are an extension of the
keyboard, not a place it should go. The focused element then never changes, so
nothing resigns and nothing has to be handed back — which matters, because the
hand-back is the direction platform-flags already records as the hard one.

The flags file gains the phone's half of that entry, and check 11.10 is the
measurement: this needs a paired hardware keyboard and there is no test on this
head that could stand in for one.

docs/platform-flags.md

@@ -168,6 +168,18 @@ that reports `GetFocus()`, the class name of the window holding it, and the page
   could fire. Not Escape, and not a bare F6: both are keys a TUI legitimately binds, and Ctrl+Shift is the
   range terminal emulators conventionally keep for themselves.
 
+**On the phone the same asymmetry arrives through a button, and the fix is one property.** The terminal's
+accessory row — Ctrl, Esc, Tab, the arrows, and the two text-size keys — is a set of ordinary Avalonia
+buttons over a `NativeWebView`. An ordinary button takes focus on tap, which takes it off the WebView, and
+`OnLostFocus` then calls the adapter's `ResignFocus()`. So pressing Tab handed the terminal one byte and
+took the keyboard away from it: everything typed afterwards on a hardware keyboard went nowhere.
+
+The symptom is what makes it worth an entry. The row goes on working — its keys are *pressed* rather than
+typed into — so what a user sees is a terminal that answers the buttons and ignores the keyboard, which
+reads as the session having died rather than as a focus problem. `Focusable = false` is what a toolbar
+button is, and it means the focused element never changes, so nothing resigns and nothing has to be handed
+back. Every button on that row carries it; check 11.10 is the measurement.
+
 None of this is covered by a test, and cannot be here: headless Avalonia has no native window, so a
 headless test renders and focuses correctly and would confirm the wrong belief. What the suite covers is
 the plumbing that drives it — that connecting asks for focus once per session, that a failed connect does