feat(cluster): opt-in leader-auto-down strategy - in-repo port of Akka AutoDowning (ScadaBridge#106)

Adds LeaderAutoDowningProvider (Host/Actors), a line-for-line port of Akka.NET
1.5.62 AutoDowning/AutoDown/AutoDownBase read from the decompiled assembly: the
leader among the reachable members downs a peer unreachable for StableAfter; a
non-leader records it as pending and downs it on becoming leader; reachable-again
or removed members are forgotten; Down/Exiting members are skipped. Its window is
carried under scadabridge.cluster.leader-auto-down.unreachable-after, so no
deprecated auto-down-unreachable-after key is emitted and Akka's boot-time
"keep-majority will be used instead" warning goes away. DownRemovalMargin reads
akka.cluster.down-removal-margin exactly as AutoDowning does (default off).

New SplitBrainResolverStrategy value "leader-auto-down", wired through BuildHocon
and ClusterOptionsValidator. The default stays "auto-down"; auto-down and
keep-oldest HOCON is byte-identical to before. Placed in Host, not
ClusterInfrastructure, per the Code Placement rule (ClusterInfrastructure stays
Akka-free).

Tests: HOCON + validator units; LeaderAutoDownTests ports Akka's AutoDownSpec
cases (leader-only, pending-on-leader-change, losing leadership, reachable again,
removed, Down/Exiting skipped, single Down per member); LeaderAutoDownFailoverTests
clones SbrFailoverTests' oldest-crash and younger-crash cases on a real two-node
cluster plus a test that Akka actually loaded the provider and its downing actor.
Mutation-checked: a provider that never calls Cluster.Down fails both failover
tests; never-downs, leader-check-dropped, reachable-ignored and pending-not-flushed
mutations each fail the unit tests.

Remaining gate before switching the default: the docker failover drill
(DRILL_MODE=active and standby mode) on leader-auto-down.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

docs/requirements/Component-ClusterInfrastructure.md

@@ -29,7 +29,7 @@ realised across two projects:
   timings, split-brain settings) bound from `appsettings.json` via the Options pattern.
 - **`src/ZB.MOM.WW.ScadaBridge.Host`** owns the cluster **bootstrap and runtime wiring**: it
   builds the Akka.NET HOCON from `ClusterOptions`, starts the `ActorSystem`,
-  configures the downing strategy (`auto-down` by default; `keep-oldest` with `down-if-alone` is the opt-in alternative), wires
+  configures the downing strategy (`auto-down` by default; `leader-auto-down`, the in-repo port, and `keep-oldest` with `down-if-alone` are the opt-in alternatives), wires
   `CoordinatedShutdown` into the service lifecycle, and provides active-node /
   cluster-membership health checks. See `Component-Host.md` (REQ-HOST-*) for detail.
 
@@ -100,6 +100,10 @@ Akka.NET cluster singletons run on the active node of their cluster and migrate
 - **The accepted trade: dual-active during a real network partition.** With both nodes alive but partitioned, each side downs the other and continues as a one-node cluster — both claim active until the partition heals and an operator restarts one side (the restarted node rejoins the other as a fresh incarnation). This trade was chosen deliberately: site pairs run one node per VM with no shared lease infrastructure (no Kubernetes, no SQL at sites) to arbitrate, and a stalled system is a bigger operational risk than a rare LAN partition.
 - **Stable-after duration**: 15 seconds of sustained unreachability before downing. This prevents premature downing during startup, rolling restarts, or transient network blips.
 
+### `leader-auto-down` — the in-repo port (ScadaBridge#106, opt-in, not yet the default)
+
+Akka deprecated `AutoDowning` in 1.5.2 and will remove it in a future release. `SplitBrainResolverStrategy: leader-auto-down` selects `LeaderAutoDowningProvider` (`src/ZB.MOM.WW.ScadaBridge.Host/Actors/LeaderAutoDowningProvider.cs`), a line-for-line port of the 1.5.62 `AutoDowning`/`AutoDown`/`AutoDownBase` logic: identical leader-only, reachable-again and removed-member semantics, and the same `StableAfter` window. `BuildHocon` emits `downing-provider-class = "ZB.MOM.WW.ScadaBridge.Host.Actors.LeaderAutoDowningProvider, ZB.MOM.WW.ScadaBridge.Host"` and carries the window under `scadabridge.cluster.leader-auto-down.unreachable-after`, so no `auto-down-unreachable-after` key is emitted and Akka's boot-time deprecation warning is not logged. The default stays `auto-down` until the docker failover drill (`DRILL_MODE=active` and standby mode) passes on the new value; switching is an appsettings flip on both nodes of a pair together, and rollback is flipping back. Plan: `docs/plans/2026-11-09-autodowning-replacement-plan.md`.
+
 ### Why not the alternatives (all verified against Akka.NET 1.5.62 source, 2026-07-21)
 
 - **keep-oldest** (used until 2026-07-21): partition-safe, but in a two-node cluster a crash of the **oldest** is a total outage. `KeepOldest.OldestDecision` only lets `down-if-alone` rescue the survivor when the surviving side has **≥ 2 members** (`otherSide == 1 && thisSide >= 2`); with 1-vs-1 the younger survivor takes `DownReachable` — it downs *itself*. Verified live on the docker rig: the survivor logged `SBR took decision Akka.Cluster.SBR.DownReachable … including myself`, exited, and looped on `InitJoin`. The strategy remains supported in `ClusterOptions` (`SplitBrainResolverStrategy: keep-oldest`) for deployments that prefer partition-safety over availability.