One emoji, one reaction: variation selectors made canonical

A heart reaction arrived as ❤ from some servers and as ❤️ (with the
emoji variation selector) from others, and a persona's own reaction kept
whatever it was given: the same emoji was two reactions, counted apart.
The selector is now dropped and put back only on a symbol that shows as
text without it (U+2000 to U+2BFF), so every heart is ❤️ and every 🔥
is 🔥. Found by the town's Hollo pair.

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

PrivaPub.Tests/Federation/ReactionTests.cs

@@ -17,6 +17,9 @@ namespace PrivaPub.Tests.Federation
 		[InlineData(":blobcat@k.example:", "blobcat")]
 		[InlineData("👍🏽", "👍🏽")]
 		[InlineData("❤️", "❤️")]
+		[InlineData("❤", "❤️")]
+		[InlineData("✌", "✌️")]
+		[InlineData("🔥\uFE0F", "🔥")]
 		[InlineData("nope", null)]
 		[InlineData("👍👍", null)]
 		[InlineData(":unknown:", null)]