Offer Use now from the finished test's own order

The fix review found Use now could still point the wrong way: the button was
worked out from per-address ranks in the devices list while the test was still
running (a better-ranked address might yet answer), and the test's successes
changed those ranks before the list was reloaded.

The test now finishes by publishing the order a reconnect on this network would
try, worked out after it has recorded where each address works, together with
the network it ran on. The page offers Use now only once the test is done, only
for that network, and only on the first address in that order that passed. The
ranks in the devices list are gone again. docs/devices.md no longer promises
where a reconnect lands: a slow address loses to a later one.

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

docs/devices.md

@@ -78,10 +78,11 @@ can add, edit, reorder and remove them without leaving the page you're on. When
 the headset reports a LAN IP on the same network as this computer and that IP
 isn't saved, it offers to add it. The offer puts the address first in the list,
 so on that network it wins over the Tailscale name; away from home the Tailscale
-name still leads. A new or edited address is tested straight away. When an address
-that would be tried before the one in use passes the test, it gets **Use now**,
-which reconnects; the reconnect picks the first address that answers, which is
-that one. Otherwise the change takes effect the next time Frame Control connects.
+name still leads. A new or edited address is tested straight away. When the test
+finishes and the first address that passed would be tried before the one in use,
+that address gets **Use now**, which reconnects. A reconnect tries the addresses
+in order and normally lands on it, but takes a later one if it's slow to answer
+then. Otherwise the change takes effect the next time Frame Control connects.
 
 ## Networks