Build: adopt HauntedPlatform 1.2.0

[remdui]

HauntedPlatform migration

Moved to HauntedPlatform

• Parent/build policy upgraded to 1.2.0.
• Shared Flatten and Enforcer configuration removed; only Platform's generic Enforcer execution remains.
• Testcontainers and Error Prone version ownership now comes from Platform; duplicate-finder and exec-plugin versions inherit Platform.

Intentionally local

• Database drivers, Paper/Velocity targets, integration and runtime acceptance tests, coverage threshold, duplicate-finder configuration, build-helper configuration, and persistence/runtime settings.

Validation

• ./mvnw clean verify
• ./mvnw dependency:tree
• ./mvnw help:effective-pom
• ./mvnw -Pplatform-acceptance clean verify reaches the Docker runtime gate locally; CI validates it with Docker.

Dependency graph

No intentional production graph change; common test/build artifacts now resolve through Platform management.