feat(scripts): self/child/parent attribute and script accessors

Phases 1+2 of the design at
docs/plans/2026-05-12-script-scope-access-design.md.

Adds ergonomic scope-aware accessors to compiled scripts. A script
on a composed TempSensor reads its own attribute via
Attributes["Temperature"]; reaches up to the parent via
Parent.Attributes["SpeedRPM"]; invokes a child script via
Children["TempSensor"].CallScript("Sample"). All resolve to the
existing flat Instance.GetAttribute / SetAttribute / CallScript
delegates by prepending the script's canonical path prefix.

Runtime types (SiteRuntime.Scripts.ScopeAccessors):
  AttributeAccessor   sync indexer + GetAsync / SetAsync
  CompositionAccessor Attributes + CallScript
  ChildrenAccessor    Children["name"] => CompositionAccessor

ScriptGlobals gains Scope, Attributes, Children, Parent properties.
Sync indexer blocks on the Instance Actor Ask; explicit GetAsync /
SetAsync are also available for callers that want to await.

Plumbing:
  - Commons.Types.Scripts.ScriptScope record (SelfPath / ParentPath).
  - ResolvedScript.Scope (defaults to ScriptScope.Root for back-compat).
  - FlatteningService emits new ScriptScope(prefix, "") for each
    composed script so a script defined on TempSensor composed under
    a parent gets SelfPath = "TempSensor".
  - ScriptActor reads the Scope from its ResolvedScript and forwards
    it through ScriptExecutionActor into ScriptGlobals on each call.

RevisionHashService not touched: the per-script canonical name
already encodes the composition path, so any structural change
already flips the hash.

10 new unit tests on the path arithmetic. Site/Template engine
suites stay green (129 + 199).

Editor surface (Phase 3: metadata fetch, Phase 4: completion +
SCADA006 / SCADA007 diagnostics) follows in the next commits.

src/ScadaLink.SiteRuntime/Actors/ScriptActor.cs

@@ -38,6 +38,7 @@ public class ScriptActor : ReceiveActor, IWithTimers
     private TimeSpan? _minTimeBetweenRuns;
     private DateTimeOffset _lastExecutionTime = DateTimeOffset.MinValue;
     private int _executionCounter;
+    private readonly Commons.Types.Scripts.ScriptScope _scope;
 
     public ITimerScheduler Timers { get; set; } = null!;
 
@@ -63,6 +64,7 @@ public class ScriptActor : ReceiveActor, IWithTimers
         _healthCollector = healthCollector;
         _serviceProvider = serviceProvider;
         _minTimeBetweenRuns = scriptConfig.MinTimeBetweenRuns;
+        _scope = scriptConfig.Scope;
 
         // Parse trigger configuration
         _triggerConfig = ParseTriggerConfig(scriptConfig.TriggerType, scriptConfig.TriggerConfiguration);
@@ -215,6 +217,7 @@ public class ScriptActor : ReceiveActor, IWithTimers
             replyTo,
             correlationId,
             _logger,
+            _scope,
             _healthCollector,
             _serviceProvider));