Building Source Han Serif fails when building HVAR: 'H' format requires 0 <= number <= 65535

I did the following steps:
1. download https://github.com/adobe-fonts/source-han-serif/blob/master/Masters/ExtraLight/OTC/VF/cidfont.VF.K.unhinted and then pass it through `tx -ufo cidfont.VF.K.unhinted el.ufo`
2. download https://github.com/adobe-fonts/source-han-serif/blob/master/Masters/Heavy/OTC/VF/cidfont.VF.K.unhinted and then pass it through `tx -ufo cidfont.VF.K.unhinted h.ufo`
3. download https://github.com/adobe-fonts/source-han-serif/blob/master/Masters/designspaces/SourceHanSerifK-VF.designspace and change source filename to `el.ufo` and `h.ufo` (referencing the UFOs from above step)
4. run `fontmake -o variable-cff2 -m SourceHanSerifK-VF.designspace --verbose DEBUG`. Observed the following error.

```
INFO:fontTools.varLib:Internal master locations:
[{'weight': 0.0}, {'weight': 1000.0}]
INFO:fontTools.varLib:Internal axis supports:
{'weight': [0.0, 0.0, 1000.0]}
INFO:fontTools.varLib:Normalized master locations:
[{'weight': 0.0}, {'weight': 1.0}]
INFO:fontTools.varLib:Index of base master: 0
INFO:fontTools.varLib:Building variable font
INFO:fontTools.varLib:Loading master fonts
INFO:fontTools.varLib:Generating fvar
INFO:fontTools.varLib:Building variations tables
INFO:fontTools.varLib:Generating avar
INFO:fontTools.varLib:Generating MVAR
INFO:fontTools.varLib:Generating HVAR
ERROR:root:In 'sourcehanserif.designspace': Generating fonts from Designspace failed: 'H' format requires 0 <= number <= 65535
Traceback (most recent call last):
File "C:\Python311\Lib\site-packages\fontmake\font_project.py", line 1208, in run_from_designspace
self._run_from_designspace_interpolatable(
File "C:\Python311\Lib\site-packages\fontmake\font_project.py", line 1326, in _run_from_designspace_interpolatable
self.build_variable_fonts(
File "C:\Python311\Lib\site-packages\fontmake\font_project.py", line 470, in build_variable_fonts
fonts = ufo2ft.compileVariableCFF2s(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\ufo2ft\__init__.py", line 295, in compileVariableCFF2s
return VariableCFF2sCompiler(**kwargs).compile_variable(designSpaceDoc)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\ufo2ft\_compilers\baseCompiler.py", line 420, in compile_variable
vfNameToTTFont = self._merge(designSpaceDoc, excludeVariationTables)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\ufo2ft\_compilers\interpolatableOTFCompiler.py", line 32, in _merge
return varLib.build_many(
^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\fontTools\varLib\__init__.py", line 1133, in build_many
vf = build(
^^^^^^
File "C:\Python311\Lib\site-packages\fontTools\varLib\__init__.py", line 1218, in build
_add_HVAR(vf, model, master_fonts, axisTags)
File "C:\Python311\Lib\site-packages\fontTools\varLib\__init__.py", line 518, in _add_HVAR
_add_VHVAR(font, axisTags, HVAR_FIELDS, getAdvanceMetrics)
File "C:\Python311\Lib\site-packages\fontTools\varLib\__init__.py", line 589, in _add_VHVAR
directSize = len(writer.getAllData())
^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\fontTools\ttLib\tables\otBase.py", line 680, in getAllData
self._doneWriting(internedTables)
File "C:\Python311\Lib\site-packages\fontTools\ttLib\tables\otBase.py", line 507, in _doneWriting
item.subWriter._doneWriting(
File "C:\Python311\Lib\site-packages\fontTools\ttLib\tables\otBase.py", line 505, in _doneWriting
items[i] = item.getCountData()
^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\fontTools\ttLib\tables\otBase.py", line 860, in getCountData
return {1: packUInt8, 2: packUShort, 4: packULong}[self.size](v)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\site-packages\fontTools\ttLib\tables\otBase.py", line 868, in packUShort
return struct.pack(">H", value)
^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: 'H' format requires 0 <= number <= 65535

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "C:\Python311\Lib\site-packages\fontmake\__main__.py", line 728, in main
project.run_from_designspace(inputs.designspace_path, **args)
File "C:\Python311\Lib\site-packages\fontmake\font_project.py", line 1224, in run_from_designspace
raise FontmakeError(
fontmake.errors.FontmakeError: In 'sourcehanserif.designspace': Generating fonts from Designspace failed: 'H' format requires 0 <= number <= 65535
```

Reproduce the failure with the Source Han Serif UFOs and designspace described in the issue, then inspect fontTools/varLib/__init__.py around _add_HVAR and _add_VHVAR, including the traceback path through otBase.py. Done means the same fontmake command generates the variable CFF2 font without the HVAR packing error.

Written by the indexing model from the issue text.