chore(platform): certify spring boot 4.1

[Sequela02]

Summary

• certify Mortar Java artifacts for Spring Boot 4.1.0 and Java 25
• prepare Java release version 0.1.0-alpha.3
• align CI, release, benchmark workflows with Java 25
• update compatibility docs, roadmap, changelog, and Spring Boot example

Verification

• .\gradlew.bat :java:spring-boot-starter:check --no-daemon
• .\gradlew.bat check --no-daemon
• .\gradlew.bat publishToMavenLocal --no-daemon --no-configuration-cache

Release

This prepares the release candidate only. Publishing still requires the guarded Release workflow with operation=publish and explicit confirmation.