P7: community flairs, from Lemmy and PieFed

- A post's flairs are read in both dialects: Lemmy 1.0's CommunityPostTag (id, slug, name, description, colour slot)
  and PieFed's lemmy:CommunityTag (display name, text and background colours, whether to blur images). Colours are
  kept only as a colour slot or a hex value.
- A community's own list comes from its `tag` and PieFed's older `lemmy:tagsForPosts`, and fills in a post that names
  only a flair's id and slug, as Lemmy's posts do. Edits and refreshes keep them current.
- Clients get them as `privapub.flairs`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub.Tests/Federation/FlairTests.cs

@@ -0,0 +1,68 @@
+using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Objects;
+
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace PrivaPub.Tests.Federation
+{
+	public class FlairTests
+	{
+		[Fact]
+		public void A_lemmy_posts_flair_is_described_by_its_community()
+		{
+			using var group = JsonDocument.Parse("""
+				{
+					"id": "https://lemmy.example/c/tenforward", "type": "Group", "preferredUsername": "tenforward", "inbox": "https://lemmy.example/c/tenforward/inbox",
+					"attributedTo": "https://lemmy.example/c/tenforward/moderators",
+					"tag": [{ "type": "CommunityPostTag", "id": "https://lemmy.example/c/tenforward/tag/news", "preferredUsername": "news",
+						"name": "News", "content": "What happened", "color": "color03" }]
+				}
+				""");
+			var community = ActorDocument.Parse(group.RootElement);
+			var page = NoteParser.Parse(JsonNode.Parse("""
+				{
+					"id": "https://lemmy.example/post/1", "type": "Page", "name": "Big news", "attributedTo": "https://lemmy.example/u/bo",
+					"tag": [{ "type": "CommunityPostTag", "id": "https://lemmy.example/c/tenforward/tag/news", "preferredUsername": "news" },
+						{ "type": "Mention", "href": "https://lemmy.example/c/tenforward", "name": "@tenforward@lemmy.example" }]
+				}
+				"""));
+
+			Assert.Equal("https://lemmy.example/c/tenforward/moderators", community.Moderators);
+			var flair = Assert.Single(ObjectShapes.Described(page.Flairs, community.Flairs));
+			Assert.Equal("News", flair.Name);
+			Assert.Equal("news", flair.Slug);
+			Assert.Equal("color03", flair.Color);
+			Assert.Equal("What happened", flair.Description);
+		}
+
+		[Fact]
+		public void Reads_piefeds_flairs_in_both_dialects_and_its_older_community_list()
+		{
+			var post = NoteParser.Parse(JsonNode.Parse("""
+				{
+					"id": "https://piefed.example/post/9", "type": "Page", "name": "A question", "attributedTo": "https://piefed.example/u/pf",
+					"tag": [{ "type": "lemmy:CommunityTag", "id": "https://piefed.example/c/ask/tag/3", "display_name": "Solved",
+						"text_color": "#ffffff", "background_color": "#2e7d32", "blur_images": false },
+						{ "type": "Hashtag", "href": "https://piefed.example/tag/q", "name": "#q" }]
+				}
+				"""));
+			using var group = JsonDocument.Parse("""
+				{
+					"id": "https://piefed.example/c/ask", "type": "Group", "preferredUsername": "ask", "inbox": "https://piefed.example/c/ask/inbox",
+					"lemmy:tagsForPosts": [{ "type": "lemmy:CommunityTag", "id": "https://piefed.example/c/ask/tag/4", "display_name": "Spoiler",
+						"text_color": "#000", "background_color": "red; background: url(x)", "blur_images": true }]
+				}
+				""");
+
+			var solved = Assert.Single(post.Flairs);
+			Assert.Equal("Solved", solved.Name);
+			Assert.Equal("#2e7d32", solved.BackgroundColor);
+			Assert.Equal("q", Assert.Single(post.Tags));
+			var spoiler = Assert.Single(ActorDocument.Parse(group.RootElement).Flairs);
+			Assert.True(spoiler.BlurImages);
+			Assert.Equal("#000", spoiler.TextColor);
+			Assert.Null(spoiler.BackgroundColor);
+		}
+	}
+}