build: Android's built-in Kotlin, the three plugins applying KGP themselves patched in third_party/

AGP 9 compiles Kotlin itself (android.builtInKotlin=true in every app),
and warns that applying the Kotlin Gradle Plugin will soon fail.
flutter_web_auth_2 5.1.0, keyboard_height_plugin 0.3.0 and flutter_tts
4.2.5 still apply it unconditionally, so their pub releases are copied
into third_party/ with only their android/build.gradle changed, and
the root overrides them by path (third_party/README.md says when to
drop each). The servers' images copy third_party/, as pub refuses an
override whose path is missing. third_party/ isn't our own code: left
out of the analysis and the format check.

All four apps' APKs build without the warning. A Kotlin daemon's
session file that slipped into Train's android/ goes, and every app's
android/.gitignore keeps .kotlin/ out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/android/.gitignore

@@ -14,3 +14,6 @@ GeneratedPluginRegistrant.java
 key.properties
 **/*.keystore
 **/*.jks
+
+# The Kotlin compiler daemon's sessions.
+.kotlin/

apps/learn/halkyon_learn_flutter/android/gradle.properties

@@ -2,5 +2,6 @@ org.gradle.jvmargs=-Xmx8G -XX:MaxMetaspaceSize=4G -XX:ReservedCodeCacheSize=512m
 android.useAndroidX=true
 # This newDsl flag was added by the Flutter template
 android.newDsl=false
-# This builtInKotlin flag was added by the Flutter template
-android.builtInKotlin=false
+# Kotlin built into the Android Gradle plugin (4 Nov 2026), every plugin
+# migrated (third_party/README.md).
+android.builtInKotlin=true