fix(siteruntime): injectable ScriptExecutionScheduler + un-leakable expression-fault test

Gitea #18: the process-wide ScriptExecutionScheduler was reached via a static
singleton (Shared) directly inside ScriptActor, ScriptExecutionActor, AlarmActor,
AlarmExecutionActor, and ScriptSchedulerStatsReporter, so it could not be
substituted — a shared mutable global for anything running more than one logical
site in a process, most visibly the test assembly. Add an optional scheduler
injection seam to all five: the Host injects nothing and gets the shared pool
(byte-for-byte unchanged), while a test (or a future multi-site host) hands an
actor its own instance. Spawning actors thread their scheduler down to the
execution children they create. Shared() now also recreates a disposed cached
instance rather than returning it (new IsDisposed guard), so a disposed scheduler
can never silently poison every later script/alarm execution.

Gitea #16: ScriptActorTests now runs on its OWN scheduler (disposed at class
teardown), so the faulted-task test can no longer strand a worker on the
process-wide pool and starve unrelated test classes (one prior run failed 22
tests). EvalGate's wait is bounded to 30 s (was unbounded — the actual leak
mechanism) and reset per test; the teardown releases one permit per worker
instead of Release(Entries), which under-released in exactly the failure case.
Full SiteRuntime suite: 6/6 runs green (524/524); was 3/6 failing on clean main.

Fixes: #18
Fixes: #16

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScriptExecutionSchedulerTests.cs

@@ -45,6 +45,24 @@ public class ScriptExecutionSchedulerTests
         Assert.Same(a, b);
     }
 
+    // Gitea #18: IsDisposed backs the Shared() recreate guard so a disposed scheduler
+    // can never be handed back and silently poison every later execution. Verified on a
+    // local instance — deliberately NOT by disposing the process-wide singleton, which
+    // would strand any script another test class queued on it (the very static hazard
+    // #18 is about).
+    [Fact]
+    public void IsDisposed_FlipsOnDispose()
+    {
+        var scheduler = new ScriptExecutionScheduler(1);
+        Assert.False(scheduler.IsDisposed);
+
+        scheduler.Dispose();
+        Assert.True(scheduler.IsDisposed);
+
+        scheduler.Dispose(); // idempotent — stays disposed, does not throw
+        Assert.True(scheduler.IsDisposed);
+    }
+
     // SiteRuntime-S2/UA5: the scheduler exposes observability gauges so a stuck /
     // saturated script-execution pool is visible to operators via site health.
     [Fact]