Reconnect the terminal view when somebody comes back to it

Going back to a terminal left alone for a while found it stuck on
"Reconnecting the terminal view…", and stuck is the right word: the banner
stayed and nothing behind it was reconnecting.

The page's whole recovery story was a setTimeout chain, and a chain is exactly
what a WebView is entitled to stop running. Chromium throttles timers in a page
nobody is looking at — down to once a minute once it has been hidden five
minutes — and a renderer that is frozen, or reclaimed and not yet reloaded,
runs none of them. So the socket drops while nobody is watching, the banner
goes up, the retry is scheduled, and the retry is then the one thing not
running.

Three defects, each of which leaves that banner up for the rest of the page's
life.

◆ NOTHING LISTENED FOR THE PAGE COMING BACK. The only thing that could clear
the banner was a timer that may never fire. terminal.js now reconnects on
visibilitychange, focus and online — the events that mean somebody is looking
again, and the ones that cannot be throttled — cancelling the pending timer and
resetting the backoff. Over a healthy socket all three do nothing, which is
what makes them safe to fire as often as clicking a window does.

◆ A HANDSHAKE THAT NEVER FINISHED WAS INVISIBLE. Every retry was scheduled by a
close or an error, so an attempt parked in CONNECTING — which is what a
suspended renderer leaves behind — scheduled nothing at all, ever. There is now
a five-second watchdog on the handshake.

◆ STALE SOCKETS SCHEDULED RETRIES, AND THAT ONE IS A LOOP RATHER THAN A STALL.
connect() never detached the old socket's handlers, and the host aborts the
displaced socket on takeover — TerminalDataPlane.UpgradeAsync, doing exactly
what it should. That close read as a fresh failure and scheduled a retry
against the socket that had just succeeded, whose own close scheduled the next:
no fixed point, reconnecting every second forever with the banner up for most
of it. Every handler now asks whether it is still the page's own attempt, and
connect() closes what it abandons.

◆ WHICH OF THE PLATFORM BEHAVIOURS ACTUALLY BIT IS NOT ESTABLISHED, and the fix
does not depend on knowing. Throttled timers, a frozen renderer and a reclaimed
one all end at the same dead timer; guessing between them would have produced a
narrower fix for one of the three.

THE TEST RUNS terminal.js ITSELF, in a fake browser, inside dotnet test.
RendererPage loads the file the shell project ships — not a transcription of its
logic into C#, which would be a copy that stays correct while the page rots —
into a Jint engine, one per test, over a harness that fakes a WebSocket and a
clock and nothing else. Jint rather than a node script because CI would run the
node one and nobody's inner loop would; the cost is that Jint is not Chromium,
so this proves the page's logic and nothing about how a WebView behaves. That
line is drawn in RendererPage's remark and picked up by two new manual checks,
1.10 for the desktop and 11.12a for the phone, which own the platform half.

Four of the nine tests fail against the page as it stood — the stale close, the
parked handshake, and the two wake-ups. Two more assert that a wake-up over a
healthy socket does nothing, and pass against either version on purpose: they
are what stops the cure being worse.

Left alone deliberately: a socket that is open and dead shows no banner at all,
because readyState still reads OPEN. That looks like a terminal that swallows
what is typed, needs a liveness probe rather than a faster retry, and is written
down at the end of 11.12a rather than quietly bundled in here.

Directory.Packages.props

@@ -221,5 +221,20 @@
       mistakes that cause real authorization holes.
     -->
     <PackageVersion Include="WireMock.Net" Version="2.13.0" />
+    <!--
+      A JavaScript interpreter, in managed code, so that terminal.js can be tested as the file that
+      ships rather than as a transcription of it into C#. Test-only and referenced by exactly one
+      project; nothing in src depends on it.
+
+      Chosen over shelling out to node, which is the obvious alternative and needs node present
+      wherever the suite runs — CI installs one already, but a test that only runs when a separate
+      command is remembered is a test that stops being run. Chosen over a headless browser for the
+      same reason several times over.
+
+      What it does not buy: Jint is not Chromium, so this proves the page's own logic and nothing
+      about how WebView2 or Android's WebView behave. That line is drawn in RendererPage's remark and
+      picked up by docs/manual-checks.md 1.10 and 11.12a.
+    -->
+    <PackageVersion Include="Jint" Version="4.16.0" />
   </ItemGroup>
 </Project>