Targets whose home page redirects are checked at /robots.txt (closes #190)

The browser follows a redirect, so the figures for the AWS Console, Akamai,
B2 and the S3 regions also timed the page it led to, for B2 and S3 on
another host. The Google Cloud Console redirects only requests that do not
say they come from another site; browsers say so and get a 403 directly.
Each of these targets is now checked at /robots.txt on its own host, which
answers without a redirect. README.md says what a figure includes. Chrome
opens a new connection for most S3 checks, so far regions now read higher.

Model: opus-5-5

README.md

@@ -413,22 +413,32 @@ Local hosts are tracked separately from WAN stats.
 
 ### Latency measurement
 
-GET requests to each target's URL as written, with `mode: 'no-cors'`,
-`cache: 'no-store'` and `referrerPolicy: 'no-referrer'`, timed with
-`performance.now()`. The Hetzner speed-test servers close the connection without
-an answer when the URL has a query string or the request has a `Referer` header.
-So no query string is added, and no `Referer` header is sent, wherever the page
-is served from. `no-store` keeps the browser's cache out of the measurement.
-Each check times out after 80% of the refresh interval (24 seconds at 30
-seconds) and is then recorded as a timeout, so a round's checks have all
-finished before the next round is due. When no WAN host answers, a recovery
-probe checks 4 WAN hosts, picked at random when it starts, every half second,
-giving up the checks it started half a second before. As soon as one answers, a
-new round starts at once, as it does after an interval change. A round started
-early gives up the last round's checks if they are still waiting, and that round
-records nothing more, so rounds never overlap. The browser chooses between IPv4
-and IPv6 for each target, as for any request; the local targets are IPv4
-addresses.
+Each check is a GET request to the target's URL as written, with
+`mode: 'no-cors'`, `cache: 'no-store'` and `referrerPolicy: 'no-referrer'`. Its
+figure is the time from sending the request until the answer's headers arrive,
+timed with `performance.now()`. Unlike ping, this includes the time the server
+takes to answer. It also includes the DNS lookup, connection setup and TLS
+whenever the browser opens a new connection. Chrome opens one for every check of
+the Hetzner speed-test servers and for most checks of the S3 regions.
+
+The browser follows every redirect, and the figure then includes the request to
+the new URL too. So a target whose home page answers with a redirect is checked
+at `/robots.txt` on the same host, which answers without one. An S3 region
+answers it with 403, and any answer counts as reachable.
+
+The Hetzner speed-test servers close the connection without an answer when the
+URL has a query string or the request has a `Referer` header. So no query string
+is added, and no `Referer` header is sent, wherever the page is served from.
+`no-store` keeps the browser's cache out of the measurement. Each check times
+out after 80% of the refresh interval (24 seconds at 30 seconds) and is then
+recorded as a timeout, so a round's checks have all finished before the next
+round is due. When no WAN host answers, a recovery probe checks 4 WAN hosts,
+picked at random when it starts, every half second, giving up the checks it
+started half a second before. As soon as one answers, a new round starts at
+once, as it does after an interval change. A round started early gives up the
+last round's checks if they are still waiting, and that round records nothing
+more, so rounds never overlap. The browser chooses between IPv4 and IPv6 for
+each target, as for any request; the local targets are IPv4 addresses.
 
 Each recorded check that fails writes one line to the browser console with
 `console.error`, and the same line to the debug log: the target's name and URL,