Fabric: register blocks/items eagerly and implement registerEntityDataSerializer via FabricEntityDataRegistry

[Arilas]

Two Fabric registration bugs found while porting and running the Fabric build in a full (~90 mod) production client instance.

1. registerBlock / registerItem register lazily, so some entries never register

FabricRegistryLoader.registerBlock/registerItem return () -> Registry.register(...) — registration only happens when somebody first polls the supplier. Consequences observed at runtime:

• Entries nobody polls during init (e.g. DTRegistries.DIRT_BUCKET) never make it into the registry at all — the dirt bucket crafting recipe then fails to load with Unknown registry key ... dynamictrees:dirt_bucket.
• A first poll after init would hit a frozen registry and throw.
• Polling the supplier twice would attempt a double registration.

Fixed by registering eagerly at call time and returning the captured instance, matching the other register* methods in the same class.

2. Missing registerEntityDataSerializer implementation — must go through FabricEntityDataRegistry

RegistryLoader declares registerEntityDataSerializer as abstract and the NeoForge loader implements it, but FabricRegistryLoader is missing the override entirely. Implementing it via vanilla's EntityDataSerializers.registerSerializer (the obvious route, and what we tried first) is rejected by Fabric API at runtime:

java.lang.IllegalStateException: Tried to register entity data serializer ... using EntityDataSerializers.registerSerializer.
This is not allowed as it can lead to desynchronization issues; use FabricEntityDataRegistry.register instead.

which fires on the first tree pack load (Species.generateSeed → Seed.<init>) and takes down the whole mod init. This PR implements the override through FabricEntityDataRegistry.register(DynamicTrees.location(name), serializer).

Compile status (full transparency)

:fabric:compileJava on develop/26.1.2 currently fails at baseline with ~587 pre-existing errors (the module looks mid-port against the 26.1 toolchain — old renderer.v1 model API, Jade/Continuity compat, loot type classes, etc.). I verified with -Xmaxerrs raised that the error set is unchanged by this patch and that none of the reported errors involve the lines added or modified here; both fixes are the exact change we run (and have validated at runtime) in our ported Fabric build.

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