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 9 rejects property interpolation inside plugins {} — the version
must be a literal String. Drop the now-unused loom_version key from
gradle.properties so it does not desync from the build script.
