A focal point is two finite numbers

float.TryParse takes "NaN" and "Infinity", and Math.Clamp keeps a NaN, so `focus=NaN,NaN` on an upload or a media PUT
was stored as given. Once the media was posted, every status and timeline response holding the post, and the Note
and its delivery, failed to serialise, for every viewer. FocalPoint.Parse takes finite numbers only (anything else is
ignored, as an unreadable focus was); the mapper and the renderer leave out a focus that isn't sound, and migration
_016 removes the ones stored before, from the uploads and from the copy each post keeps. What PrivaPub sends is
unchanged for any focus that could be sent before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Http/MastodonMediaTests.cs

@@ -136,6 +136,25 @@ namespace PrivaPub.Tests.Http
 			Assert.Null((await alice.Client.Put($"/api/v1/media/{id}", ("description", ""))).Ok().Body.Text("description"));
 		}
 
+		// a focal point is two finite numbers: NaN or Infinity is ignored, never stored (it broke every timeline holding the post)
+		[Fact]
+		public async Task A_focal_point_that_is_not_a_number_is_ignored()
+		{
+			var alice = await _host.Mastodon("alice");
+			var uploaded = (await Upload(alice, "/api/v2/media", MastodonHelpers.JpegWithMetadata(32, 32), "image/jpeg", "a.jpg", ("focus", "NaN,NaN"))).Ok();
+			Assert.Null(uploaded.Body["meta"]?["focus"]);
+			var id = uploaded.Body.Text("id");
+
+			(await alice.Client.Put($"/api/v1/media/{id}", ("focus", "0.5,0.25"))).Ok();
+			foreach (var unsound in new[] { "NaN,0", "Infinity,1", "0,-Infinity" })
+			{
+				var focus = (await alice.Client.Put($"/api/v1/media/{id}", ("focus", unsound))).Ok().Body["meta"]!["focus"]!;
+				Assert.Equal((0.5, 0.25), (focus["x"]!.GetValue<double>(), focus["y"]!.GetValue<double>()));
+			}
+			var status = (await alice.Client.Post("/api/v1/statuses", ("status", "focused"), ("media_ids[]", id))).Ok();
+			Assert.Equal(0.5, status.Body["media_attachments"]![0]!["meta"]!["focus"]!["x"]!.GetValue<double>());
+		}
+
 		[Fact]
 		public async Task Unsupported_unreadable_and_missing_files_are_refused_with_422()
 		{