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.

tests/ScadaLink.SiteRuntime.Tests/Scripts/ScopeAccessorTests.cs

@@ -0,0 +1,87 @@
+using ScadaLink.Commons.Types.Scripts;
+using ScadaLink.SiteRuntime.Scripts;
+
+namespace ScadaLink.SiteRuntime.Tests.Scripts;
+
+/// <summary>
+/// Phase 1 of the script-scope rollout: verify path arithmetic for the new
+/// Attributes / Children / Parent accessors. The actor-mediated reads/writes
+/// are exercised end-to-end in Phase 2 once flattening carries scope info.
+/// </summary>
+public class ScopeAccessorTests
+{
+    [Fact]
+    public void Root_SelfPath_Empty()
+    {
+        Assert.Equal("", ScriptScope.Root.SelfPath);
+        Assert.Null(ScriptScope.Root.ParentPath);
+        Assert.False(ScriptScope.Root.HasParent);
+    }
+
+    [Fact]
+    public void CompositionScope_HasParent()
+    {
+        var scope = new ScriptScope("TempSensor", "");
+        Assert.True(scope.HasParent);
+        Assert.Equal("", scope.ParentPath);
+    }
+
+    [Fact]
+    public void AttributeAccessor_RootScope_ResolvesBareKey()
+    {
+        var acc = new AttributeAccessor(null!, "");
+        Assert.Equal("Temperature", acc.Resolve("Temperature"));
+    }
+
+    [Fact]
+    public void AttributeAccessor_ComposedScope_PrependsPath()
+    {
+        var acc = new AttributeAccessor(null!, "TempSensor");
+        Assert.Equal("TempSensor.Temperature", acc.Resolve("Temperature"));
+    }
+
+    [Fact]
+    public void AttributeAccessor_NestedScope_ChainsPath()
+    {
+        var acc = new AttributeAccessor(null!, "Motor.TempSensor");
+        Assert.Equal("Motor.TempSensor.Temperature", acc.Resolve("Temperature"));
+    }
+
+    [Fact]
+    public void CompositionAccessor_AttributesShareScope()
+    {
+        var comp = new CompositionAccessor(null!, "TempSensor");
+        Assert.Equal("TempSensor", comp.Path);
+        Assert.Equal("TempSensor", comp.Attributes.ScopePrefix);
+    }
+
+    [Fact]
+    public void CompositionAccessor_ResolveScript_PrependsPath()
+    {
+        var comp = new CompositionAccessor(null!, "TempSensor");
+        Assert.Equal("TempSensor.Sample", comp.ResolveScript("Sample"));
+    }
+
+    [Fact]
+    public void CompositionAccessor_EmptyPath_LeavesScriptNameBare()
+    {
+        var comp = new CompositionAccessor(null!, "");
+        Assert.Equal("Sample", comp.ResolveScript("Sample"));
+    }
+
+    [Fact]
+    public void ChildrenAccessor_FromRoot_GivesUnpathedChild()
+    {
+        var children = new ChildrenAccessor(null!, "");
+        var temp = children["TempSensor"];
+        Assert.Equal("TempSensor", temp.Path);
+    }
+
+    [Fact]
+    public void ChildrenAccessor_FromComposition_PrefixesChild()
+    {
+        var children = new ChildrenAccessor(null!, "Motor");
+        var temp = children["TempSensor"];
+        Assert.Equal("Motor.TempSensor", temp.Path);
+    }
+}