Latency statistics written once, thresholds read from CONFIG (closes #102)

A target's min, max, median and average latency now come from one list
of its answers, through latencyStats(), which the summary's figures use
too, so the median is written once. latencyHex() and latencyClass() read
one table of color limits in CONFIG. The health thresholds, the debug
log's length, the gateway check's timeout, the recovery probe's number
of hosts and interval, how often the rows are sorted and the delay
before the first sparkline resize are CONFIG entries, read where the
numbers were. minLatency() and its three siblings stay for the unit
tests. Nothing the page does or shows changes. A unit test now covers
the summary's figures.

Model: opus-5-5

test/unit/main.test.js

@@ -360,6 +360,30 @@ for (const { history, latencies, statistics } of [
     });
 }
 
+// The summary's figures come from each WAN target's last check, by the same
+// rules as a target's own: here four answered, one was found unreachable
+// and the rest have not been checked yet. The median, 22.5, and the
+// average, 21.25, are rounded.
+test("the summary's min, max, median and average latency over the WAN targets' last checks", () => {
+    const state = new AppState([]);
+    [30, 10, null, 25, 20].forEach((latency, i) =>
+        state.wan[i].pushSample(
+            Date.now(),
+            latency === null
+                ? { latency: null, error: "unreachable" }
+                : { latency, error: null },
+        ),
+    );
+    assert.deepEqual(state.wanStats(), {
+        reachable: 4,
+        total: state.wan.length,
+        min: 10,
+        max: 30,
+        med: 23,
+        avg: 21,
+    });
+});
+
 // An app state in which, of the WAN targets, the first timedOut timed out,
 // the next unreachable were found unreachable, the next answered answered
 // after latency ms, and the rest have not been checked yet.