Fix DimPos being persisted under a hardcoded NBT key

[rubensworks]

Problem

The ValueInput/ValueOutput migration left the DimPos NBT type in NBTClassTypesNeoForge asymmetric — it writes to a hardcoded "dim" child but reads a child named after the field:

public void writePersistedField(String name, DimPos object, ValueOutput tag) {
    ValueOutput dimPos = tag.child("dim");   // <- hardcoded
    ...
}

public DimPos readPersistedField(String name, ValueInput tag) {
    ValueInput dimPos = tag.child(name).orElseThrow();   // <- by field name
    ...
}

So no persisted DimPos ever round-trips; reading always throws. master-1.21-lts and earlier are correct (tag.put(name, dimPos) / tag.getCompound(name)), so this is a regression limited to the ValueOutput-based branches. A second consequence: two DimPos fields written into the same tag under different names clobber each other.

This is what crashes IntegratedDynamics clients in CyclopsMC/IntegratedDynamics#1703, via ValueTypeListProxyPositioned:

java.util.NoSuchElementException: No value present
	at java.base/java.util.Optional.orElseThrow(Optional.java:377)
	at NBTClassTypesNeoForge$2.readPersistedField(NBTClassTypesNeoForge.java:52)
	at NBTClassTypesNeoForge$2.readPersistedField(NBTClassTypesNeoForge.java:39)
	at NBTClassType.readNbt(NBTClassType.java:498)
	at ValueTypeListProxyPositioned.readGeneratedFieldsFromNBT(...)

PositionedOperator and anything else persisting a DimPos hit the same thing.

Changes

• Write to tag.child(name) instead of tag.child("dim").
• Reading falls back to the old hardcoded "dim" key when the named child is absent, so data already written by affected versions stays readable instead of throwing.

Testing

loader-neoforge's own build does not resolve here (the commoncapabilities API dependency is unreachable from this environment, and it fails identically on a clean checkout), so I type-checked the changed file against the locally built loader-common classes plus the patched Minecraft jar, and drove a round-trip through TagValueOutput/TagValueInput:

Before:

serialized keys: [dim]
Exception in thread "main" java.util.NoSuchElementException: No value present
	at NBTClassTypesNeoForge$2.readPersistedField(NBTClassTypesNeoForge.java:52)

(the reporter's exact frames)

After:

serialized keys: [pos]
read: minecraft:overworld BlockPos{x=1, y=2, z=3}
legacy read: minecraft:overworld BlockPos{x=1, y=2, z=3}
OK

where legacy read feeds in a tag whose DimPos sits under the old "dim" key.

Branch

Affected branches are master-1.21.8, master-1.21.10, master-26-lts and master-26. The two 1.21.x branches look dormant (last commits Nov/Dec 2025), so this targets master-26-lts for upmerging — happy to retarget if you want it lower.

---

Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[coveralls]

coverage: 31.028% (-0.004%) from 31.032% — claude/cyclopscore-dimpos-nbt-key into master-26-lts