Coordinator serves the last-good snapshot within a staleness threshold

WHY: any single typed fetch failure was a failed pass, so the whole
board flipped unavailable on one dropped poll and stayed dark until the
API answered again — even though HA's coordinator machinery keeps the
last good data on a failed pass and the panel could safely keep
rendering it for a while.

The coordinator now caches every successful snapshot with its
time.monotonic stamp; while the elapsed time since the last success is
within the configured staleness threshold (CONF_SNAPSHOT_STALENESS,
seconds, default 3x the update interval = 900 s) a typed failure serves
that cache instead: the panel keeps rendering last-good data, entities
stay available, and the outage is logged as a clear warning.  Past the
threshold — or before the first success — the pass fails as before
(typed error logged and chained into UpdateFailed); a later successful
fetch refreshes the cache and the entities recover with no config-entry
reload.  The threshold rides the options flow (validated as a positive
int at least the update interval, with a dedicated error string) and
the strings files; the clock is read through a _monotonic seam so the
staleness tests drive it deterministically.

custom_components/nestquest/core/const.py

@@ -81,6 +81,21 @@ CONF_UPDATE_INTERVAL = "update_interval"
 DEFAULT_UPDATE_INTERVAL = 300
 MIN_UPDATE_INTERVAL = 30
 
+# Last-good snapshot staleness (Feature 18), in SECONDS: how long the
+# coordinator keeps SERVING the last good snapshot after the API stops
+# answering.  A failed poll whose elapsed time since the last successful
+# fetch is within the threshold returns the cached snapshot (the panel
+# keeps rendering last-good data, entities stay available); past it the
+# poll fails the HA way and the entities go unavailable.  The default is
+# 3x the default update interval (900 s = 15 minutes): three consecutive
+# failed five-minute polls before the board is declared stale — enough
+# to ride out an API service restart, short enough not to render a
+# days-old board all day.  Validation floor: the threshold must be at
+# least the configured update interval, or the cache could never span
+# even one failed poll.
+CONF_SNAPSHOT_STALENESS = "snapshot_staleness"
+DEFAULT_SNAPSHOT_STALENESS = 3 * DEFAULT_UPDATE_INTERVAL
+
 # Notification automations (Feature 11).  The notify target is a free-text
 # Home Assistant service name (e.g. "notify.mobile_app_dads_phone") the
 # household fills in — never a hard-coded personal target; the times are