Fix erosion sync flooding on player login

[kaiserproger]

What this fixes

Closes issue #63.

On large worlds, sendFullSyncToPlayer() sends every persisted erosion chunk to a player during login. In our production world this produced 100k+ trmt:main packets immediately after joining, delaying vanilla chunk data and keepalives enough to cause empty-world/void symptoms and Timed out disconnects.

A packet trace from the failing case showed ~102k TRMT payloads and ~132 MB received while zero ClientboundLevelChunkWithLightPacket and zero ClientboundKeepAlivePacket reached the client.

Change

• Stop sending the world-wide erosion sync from PlayerLoggedInEvent.
• Sync erosion data when Forge fires ChunkWatchEvent.Watch, so the client only receives data for chunks it actually tracks.
• Send an empty chunk sync when there is no erosion data so stale client cache entries are cleared on re-watch.

We tested the same approach on the affected server and the player could join normally afterward.

I have read the Contributor License Agreement in CLA.md and I agree to its terms.

Verification

gradle build passes on the 1.20.1-forge branch with Forge 47.4.10 / Java 17.