Switch to Yarn mappings 26.2+build.3

Loom rejected officialMojangMappings() with a non-obfuscated
environment error, and Mojang client_mappings are not published for
26.2 yet. Pin Yarn at 26.2+build.3 via the new yarn_mappings property
and rewrite the client entrypoint with Yarn names: MinecraftClient,
Text, Formatting, inGameHud.getChatHud(), getSession().getUsername().

gradle.properties

@@ -4,6 +4,7 @@ org.gradle.configuration-cache=false
 
 # Minecraft 26.2 released on 2026-06-16.
 minecraft_version=26.2
+yarn_mappings=26.2+build.3
 loader_version=0.19.3
 fabric_version=0.152.2+26.2