Assertion `(char *) aligned_storage + holder_size <= (char *)base_storage + base_allocation' failed.

Hi,

Many thanks for the great library!

We just migrated to boost 1.78 and start getting:
```
python3: libs/python/src/object/class.cpp:762: static void* boost::python::instance_holder::allocate(PyObject*, std::size_t, std::size_t, std::size_t): Assertion `(char *) aligned_storage + holder_size <= (char *)base_storage + base_allocation' failed.
```
on arm platform. On x64 everything works perfect.

I took a look into boost::python::instance_holder::allocate with debugger and see:
```
sizeof(alignment_marker_t) == 4
alignment == 4
holder_size == 16
```
so assertion is correct.

I'm a bit confused, why we need -1 at https://github.com/boostorg/python/blame/develop/src/object/class.cpp#L750?

Looking forward for your answer!

Start in libs/python/src/object/class.cpp around the allocation logic near line 750 and inspect how the -1 adjustment interacts with ARM alignment and storage sizes. Reproduce the assertion on an ARM platform and compare it with the x64 behavior. Done means establishing whether the allocation calculation is incorrect and documenting or correcting it with validation for both platforms.

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