perf(sitelog): sampled per-run events; interval run summaries; site_events replication policy pinned

Implements WP3.2 stage (b) per docs/plans/2026-08-15-site-events-policy-design.md.

- Per-run instance-script Started/Completed Info site events are now off by
  default (SiteRuntimeOptions.PerRunScriptEvents=false) instead of firing on
  every run, closing the dominant site_events writer. Gated at the ScriptRunLauncher
  call sites (moved there from ScriptExecutionActor by WP3.1). Error-level events
  (timeout/failure/stuck-watchdog/recursion-limit) remain unconditional.
- ScriptRunSummaryRecorder accumulates per-(instance, script) run counters and a
  new site-only ScriptRunSummaryFlushService emits one aggregate "script" Info
  site event per ScriptRunSummaryIntervalSeconds (default 300s), top-50-script
  breakdown with an "others" rollup, zero-activity intervals emit nothing.
- Per-script opt-in via PerRunScriptEventScripts ("Instance/Script" exact or
  "Instance/*" wildcard), matched by the new pure ScriptRunEventPolicy. All three
  options are read from IOptionsMonitor<SiteRuntimeOptions> per run, so the
  policy is hot-togglable without a restart.
- Fixed the stale "event log is not replicated" comment at AkkaHostedService.cs
  (~905): site_events IS registered in SiteLocalDbSetup.ReplicatedTables — the
  singleton is what makes queries always hit the actively-written copy;
  replication is what gives the singleton history to read after a failover
  (memo Decision (b)). site_events replication itself is unchanged (still
  registered) and already pinned by
  tests/ZB.MOM.WW.ScadaBridge.Host.Tests/SiteLocalDbCdcRegistrationTests.cs.
- Updated Component-SiteEventLogging.md (Volume Policy section, corrected
  Storage/replication rationale) and Component-SiteRuntime.md (Script Run
  Launch + Error Handling sections).

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/TestSupport/TestOptionsMonitor.cs

@@ -0,0 +1,29 @@
+using Microsoft.Extensions.Options;
+
+namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests.TestSupport;
+
+/// <summary>
+/// Minimal <see cref="IOptionsMonitor{TOptions}"/> test double with a mutable
+/// <see cref="Set"/> so a test can flip a value mid-run and assert the NEXT read observes it
+/// — used by the WP3.2 hot-toggle tests
+/// (<c>SiteRuntimeOptions.PerRunScriptEvents</c>/<c>PerRunScriptEventScripts</c> are read via
+/// <c>IOptionsMonitor.CurrentValue</c> per run specifically so they don't need a restart).
+/// Avoids depending on <c>Microsoft.Extensions.Configuration</c>'s reload-token plumbing,
+/// which is awkward to drive deterministically from xUnit.
+/// </summary>
+public sealed class TestOptionsMonitor<T>(T initial) : IOptionsMonitor<T>
+{
+    private T _current = initial;
+
+    /// <inheritdoc />
+    public T CurrentValue => _current;
+
+    /// <inheritdoc />
+    public T Get(string? name) => _current;
+
+    /// <inheritdoc />
+    public IDisposable? OnChange(Action<T, string?> listener) => null;
+
+    /// <summary>Replaces the current value, observed by the next <see cref="CurrentValue"/> read.</summary>
+    public void Set(T value) => _current = value;
+}