Interval change keeps unreported samples for the reporter (closes #124)

The interval menu cleared every host's history, and the reporter builds
each report from that history, so the samples recorded since the last
delivered report were never sent. The menu now calls
HostState.clearHistory, which moves the history's entries to
clearedHistory, keeping the last maxHistoryPoints as the history does;
buildReport reads both. The per-host marks are unchanged, so a sample a
delivered report had is not sent again, and the page's history still
starts over. Reporter is exported for the new unit test.

Model: opus-5-5

README.md

@@ -142,9 +142,11 @@ same-origin path `/api/v1/reports`: a random per-browser `clientId` kept in
 `localStorage`, `geo` sent as null, and each host's unreported, non-paused
 samples (timestamp, latency, error). A per-host high-water mark makes every
 report a delta, so only new samples are sent; the mark advances only on a
-delivered report, and while paused nothing is sent. Delivery failure is quiet —
-one debug-log line per outage, retried at the next interval, never blocking
-probing. The report-building step is a pure function of host state.
+delivered report, and while paused nothing is sent. Changing the interval clears
+each host's history on the page; the samples in it not yet reported still go in
+the next report. Delivery failure is quiet — one debug-log line per outage,
+retried at the next interval, never blocking probing. The report-building step
+is a pure function of host state.
 
 ### Backend