Add whole-release embeds to FramePlayer and un-gate the release embed share affordance

The queue gains an armed-but-idle state (Arm/Start) so a release embed stages track 0 prerender-safe, then queues the full release on first play and auto-advances.

DeepDrftTests/EmbedSnippetBuilderTests.cs

@@ -0,0 +1,54 @@
+using DeepDrftPublic.Client.Helpers;
+
+namespace DeepDrftTests;
+
+/// <summary>
+/// Unit tests for the share-popover embed snippet (<see cref="EmbedSnippetBuilder"/>). The builder is
+/// the mode-aware half of SharePopover: track mode targets FramePlayer's TrackEntryKey param, release
+/// mode targets its ReleaseEntryKey param. The iframe chrome (dimensions, autoplay) must be identical
+/// across both. Pure string composition, tested directly without rendering the component.
+/// </summary>
+[TestFixture]
+public class EmbedSnippetBuilderTests
+{
+    private const string BaseUri = "https://deepdrft.example/";
+
+    [Test]
+    public void ForTrack_EmitsTrackEntryKeySrc()
+    {
+        var snippet = EmbedSnippetBuilder.ForTrack(BaseUri, "abc123");
+
+        Assert.That(snippet, Does.Contain(@"src=""https://deepdrft.example/FramePlayer?TrackEntryKey=abc123"""));
+        Assert.That(snippet, Does.Not.Contain("ReleaseEntryKey"));
+    }
+
+    [Test]
+    public void ForRelease_EmitsReleaseEntryKeySrc()
+    {
+        var snippet = EmbedSnippetBuilder.ForRelease(BaseUri, "rel-xyz");
+
+        Assert.That(snippet, Does.Contain(@"src=""https://deepdrft.example/FramePlayer?ReleaseEntryKey=rel-xyz"""));
+        Assert.That(snippet, Does.Not.Contain("TrackEntryKey"));
+    }
+
+    [Test]
+    public void BothModes_ShareIdenticalIframeChrome()
+    {
+        var track = EmbedSnippetBuilder.ForTrack(BaseUri, "k");
+        var release = EmbedSnippetBuilder.ForRelease(BaseUri, "k");
+
+        Assert.Multiple(() =>
+        {
+            foreach (var snippet in new[] { track, release })
+            {
+                Assert.That(snippet, Does.StartWith("<iframe "));
+                Assert.That(snippet, Does.Contain(@"width=""656"""));
+                Assert.That(snippet, Does.Contain(@"height=""196"""));
+                Assert.That(snippet, Does.Contain(@"frameborder=""0"""));
+                Assert.That(snippet, Does.Contain(@"style=""border-radius:8px;"""));
+                Assert.That(snippet, Does.Contain(@"allow=""autoplay"""));
+                Assert.That(snippet, Does.EndWith("></iframe>"));
+            }
+        });
+    }
+}