fix(historian): capture while the recording Primary is unreachable (#672)

A SIGKILLed Primary stays an unreachable-but-not-downed member until
auto-down, and the redundancy singleton usually dies with it, so the
standby's cached snapshot kept naming it Primary and nobody captured for
~30 s. DriverHostActor now holds its own ReachabilityEvent (+MemberUp)
subscription, caches the last snapshot, and excludes unreachable peers
from the historization peer-is-Primary predicate only, republishing just
the capture verdict on a reachability change. Alarm drain and the Primary
data-plane verdict are unchanged. Gated by
ContinuousHistorization:CaptureWhilePrimaryUnreachable (default true).

Tests: message-driven cases with real ReachabilityEvents (incl. a
falsification control on the drain/data-plane verdicts) plus a real
two-node cluster crash test with downing disabled.

src/Server/ZB.MOM.WW.OtOpcUa.Host/appsettings.json

@@ -55,7 +55,9 @@
     "MaxWriteAttempts": 5,
     "_MaxWriteAttemptsComment": "Gitea #538: consecutive failed drain passes one tag may accumulate — counted only across passes where OTHER tags wrote successfully — before its queued entries are abandoned so the FIFO head can advance.",
     "MinBackoffSeconds": 1,
-    "MaxBackoffSeconds": 30
+    "MaxBackoffSeconds": 30,
+    "CaptureWhilePrimaryUnreachable": true,
+    "_CaptureWhilePrimaryUnreachableComment": "Gitea #672: a standby stood down for its mesh Primary resumes capture as soon as that Primary is reported UNREACHABLE (not after auto-down) — shrinks a crash-failover capture gap to the failure-detector window, at the cost of short duplicate windows. false = pre-#672."
   },
   "AlarmHistorian": {
     "_comment": "Durable store-and-forward alarm sink. Buffers into the node's consolidated LocalDb (see the LocalDb section) so the queue replicates to the redundant pair peer, and drains alarm events to the ServerHistorian gateway's SendEvent path from whichever node holds the Primary role. The downstream connection (endpoint/key/TLS) is sourced from the ServerHistorian section.",