🚧 Add Paper loading and Spigot fallback

[NoltoxGit]

Summary

Fix the startup failure caused by zEssentials enabling before zMenu registers its services.

• Add paper-plugin.yml with required zMenu and PlaceholderAPI dependencies, explicit load ordering and classpath access.
• Preserve MariaDB 3.5.6 through a native Paper loader using a Maven Central mirror.
• Adapt the existing dynamic command registration for native Paper plugins.
• Add an independent Spigot fallback with console warnings and permission-gated administrator notifications.

The fallback follows the approach introduced in Maxlego08/zMenu@d2382a9. It provides no functional zEssentials features on Spigot.

Validation

• Built locally using the separate Gradle/Java 25 correction (⬆️ Update Gradle to 9.8.1 and fix Java 25 builds #249).
• Inspected the final JAR: both descriptors, expanded versions and entry points are present.
• Tested on Paper 26.3 / Java 25 with a locally built zMenu 1.1.1.9: service availability, commands, menu loading, reload and shutdown passed.
• Tested the same JAR on Spigot 26.3: only the inactive fallback enabled, with no functional commands.

This change does not add Minecraft 26.3 NMS implementations. Player-visible administrator notifications have not been tested with a connected client.

Integration order

Merge the #249 correction first.