Support profile-specific fixture suites

[GhostTypes]

Summary

• split checked-in fixture jars into 26x and legacy profiles and run CI as a profile matrix
• remove release downloader tooling and wire Gradle/tests to profile-specific fixture dirs
• update stub generation to emit JVM nested classes as nested Java source types, fixing MCP SDK stubs
• add legacy compatibility shims for the expanded fixture set and update Spotless/google-java-format for JDK 25 with ratcheting

Verification

• .\gradlew spotlessCheck --no-daemon
• .\gradlew test -PautoGenerateStubs -PparserProfile=legacy --no-daemon
• .\gradlew test -PautoGenerateStubs -PparserProfile=26x --no-daemon