feat(scripts): add the Alarms.CurrentAsync() read accessor for site scripts

MES alarm-status API §5.2 (docs/plans/2026-06-30-mes-alarm-status-api.md,
Phase 1 tasks 2-4). Site `Call` scripts had NO way to read alarm condition
state: the `Alarm` global exists only inside an on-trigger handler and
describes the one alarm that fired, and native mirrored conditions were
reachable only from the Debug View. That gap blocked the CvdReactor
SimpleAlarmStatus/AlarmStatus scripts entirely -- they cannot be written
without it. `Alarms.CurrentAsync()` closes it.

The data was already local: the script runs inside its own Instance Actor's
context, so this is a LOCAL Ask -- the same mechanism attribute reads use, no
cross-cluster hop. A dedicated GetAlarmSnapshotRequest/Response is used rather
than reusing DebugSnapshotRequest, which would materialise every attribute
value on every alarm poll; both are served from the same
BuildAlarmStatesSnapshot(), so the script view and the operator's Debug View
can never disagree.

Deliberate shape decisions:
  - NOT scope-prefixed, unlike Attributes. Alarm identity is not a
    scope-relative attribute name (computed alarms are keyed by configured
    name, native conditions by a source-supplied reference), so prefixing
    would hand a composed script a silently truncated list.
  - Read-only. Native alarms are a read-only mirror of the source (no
    ack-back), so no acknowledge/shelve operation is exposed.
  - Placeholder rows are NOT pre-filtered: a caller must be able to tell
    "binding configured and quiet" from "binding unknown". The documented
    filter is `Active && !IsConfiguredPlaceholder`.
  - ScriptAlarm lives in Commons so the runtime accessor and the compile-only
    surface project to the SAME type -- a script binding at the design-time
    gate binds identically at the site. Condition is the authority for
    active/acked/severity, so one filter expression works across computed and
    native alarms.

Mirrored on BOTH design-time surfaces. ScriptCompileSurface is covered by the
reflection parity guard (AlarmsAccessor added to its mirror pairs). The Central
UI Test-Run SandboxScriptHost is the third, hand-maintained mirror that the
parity test cannot reach (Central UI does not reference Site Runtime); without
it the design page would false-flag CS1061 on scripts the deploy gate accepts.
It throws a labelled ScriptSandboxException at run time -- there is no central
route to per-instance alarm state, and returning an empty list would read as
"nothing is in alarm", which is worse than an error.

ScriptTrustPolicy needs NO change, and the reason is structural rather than
incidental: the trust boundary is a deny-list over API roots, not an allow-list
of context members. A test pins that no ForbiddenScopes entry prefixes the
Commons script-surface namespace, so a future deny-list entry cannot silently
make ScriptAlarm untouchable.

Tests: 6 accessor cases (Ask contract, full native projection incl. AckTime,
unacked, computed-alarm derivation, placeholder visibility, scope-independence),
2 InstanceActor snapshot cases incl. equality with the Debug View row set, the
full MES script shape compiling against ScriptCompileSurface, 2 trust cases,
and a sandbox diagnose-clean case reading every projected ScriptAlarm field.

src/ZB.MOM.WW.ScadaBridge.CentralUI/ScriptAnalysis/SandboxScriptHost.cs

@@ -79,6 +79,36 @@ public class SandboxScriptHost
     /// </summary>
     public SandboxCompositionAccessor? Parent =>
         Scope.ParentPath == null ? null : new SandboxCompositionAccessor(Instance, Scope.ParentPath);
+
+    /// <summary>
+    /// Read-only accessor for the instance's current alarm conditions, mirroring the site
+    /// runtime's <c>Alarms</c> global (MES alarm-status API §5.2). Present so a script using
+    /// <c>Alarms.CurrentAsync()</c> BINDS in the Test Run editor — without it the editor
+    /// would show a spurious CS1061 on a script that deploys and runs perfectly.
+    /// </summary>
+    public SandboxAlarmsAccessor Alarms { get; } = new();
+}
+
+/// <summary>
+/// Sandbox mirror of <c>ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts.AlarmsAccessor</c>.
+/// Exists for editor/compile parity only: alarm state is held per-instance by the site's
+/// Instance Actor and there is no central Test Run route to it, so an actual call throws a
+/// clearly-labelled <see cref="ScriptSandboxException"/> rather than quietly returning an
+/// empty list that would read as "nothing is in alarm".
+/// </summary>
+public class SandboxAlarmsAccessor
+{
+    /// <summary>
+    /// Mirrors <c>AlarmsAccessor.CurrentAsync</c> for compile parity; unsupported at run
+    /// time in the central Test Run sandbox.
+    /// </summary>
+    /// <param name="cancellationToken">Token used to cancel the read (unused in the sandbox).</param>
+    /// <returns>Never returns; always throws <see cref="ScriptSandboxException"/> in the sandbox.</returns>
+    public Task<IReadOnlyList<ScriptAlarm>> CurrentAsync(CancellationToken cancellationToken = default)
+        => throw new ScriptSandboxException(
+            "Alarms.CurrentAsync() reads the site Instance Actor's live alarm conditions, " +
+            "which aren't available in the central Test Run sandbox — deploy to a site to " +
+            "exercise alarm-reading scripts.");
 }
 
 /// <summary>