fix(audience-sample): grant SampleApp.Tests access to internal SSOT and add missing Css.Last

Three latent compile errors in the sample-app Tests assembly that
the dotnet test runner did not surface (those tests run only under
Unity Test Runner) but Unity flagged on first compile, plus the
missing .meta files Unity asks for on next project open.

- SDK Runtime AssemblyInfo.cs grants InternalsVisibleTo to "Immutable.Audience.Samples.SampleApp.Tests" so the live-fire fixture can read AudiencePaths, EventNames, EventPropertyKeys, MessageFields, and the rest of the centralised SSOT directly instead of hand-rolling wire strings.
- New AssemblyInfo.cs in the sample-app Scripts assembly grants the same access so SampleAppUi, SampleAppCustomEvents, and SampleAppCustomEventPropertyKeys are reachable from the live-fire and UXML-alignment fixtures.
- SampleAppUi.Css gains the Last constant ("last"), referenced by AudienceSample.UI when marking the trailing actions row in a typed-event accordion.
- Adds the missing .meta files Unity flags on first open: README, Tests/Editor and DeviceCollectorTests, Tests/Runtime/MessagesTests and WireFixture, Tests/Runtime/SampleAppUxmlAlignmentTests, and the new sample-app AssemblyInfo.

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/Scripts/AssemblyInfo.cs

@@ -0,0 +1,4 @@
+using System.Runtime.CompilerServices;
+
+// Grants test access without exposing these helpers on the SDK consumer surface.
+[assembly: InternalsVisibleTo("Immutable.Audience.Samples.SampleApp.Tests")]

examples/audience/Assets/SampleApp/Scripts/AssemblyInfo.cs.meta

@@ -246,6 +246,10 @@ internal static class Css
             internal const string FieldLabel = "field-label";
             internal const string Actions = "actions";
 
+            // Marks the trailing actions row in a typed-event accordion so
+            // the USS rule can drop the bottom border on the last entry.
+            internal const string Last = "last";
+
             // Status-cell click-to-copy hook.
             internal const string StatusValue = "status-value";
 

examples/audience/Assets/SampleApp/Scripts/SampleAppUi.cs

@@ -7,3 +7,6 @@
 // JsonReader.DeserializeObject; both stay internal so their
 // signatures aren't frozen into the public API.
 [assembly: InternalsVisibleTo("Immutable.Audience.Samples.SampleApp")]
+
+// SampleApp tests pin against the same internal constants the runtime SDK uses.
+[assembly: InternalsVisibleTo("Immutable.Audience.Samples.SampleApp.Tests")]