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/ScriptExecutionActor.cs

@@ -33,6 +33,7 @@ public class ScriptExecutionActor : ReceiveActor
         IActorRef replyTo,
         string correlationId,
         ILogger logger,
+        Commons.Types.Scripts.ScriptScope scope,
         ISiteHealthCollector? healthCollector = null,
         IServiceProvider? serviceProvider = null)
     {
@@ -43,7 +44,7 @@ public class ScriptExecutionActor : ReceiveActor
         ExecuteScript(
             scriptName, instanceName, compiledScript, parameters, callDepth,
             instanceActor, sharedScriptLibrary, options, replyTo, correlationId,
-            self, parent, logger, healthCollector, serviceProvider);
+            self, parent, logger, scope, healthCollector, serviceProvider);
     }
 
     private static void ExecuteScript(
@@ -60,6 +61,7 @@ public class ScriptExecutionActor : ReceiveActor
         IActorRef self,
         IActorRef parent,
         ILogger logger,
+        Commons.Types.Scripts.ScriptScope scope,
         ISiteHealthCollector? healthCollector,
         IServiceProvider? serviceProvider)
     {
@@ -102,7 +104,8 @@ public class ScriptExecutionActor : ReceiveActor
                 {
                     Instance = context,
                     Parameters = new ScriptParameters(parameters ?? new Dictionary<string, object?>()),
-                    CancellationToken = cts.Token
+                    CancellationToken = cts.Token,
+                    Scope = scope
                 };
 
                 var state = await compiledScript.RunAsync(globals, cts.Token);