GeezerVison

`count_members()` now raises `SystemError` (returning `-1`) when a struct
sequence descriptor places an unnamed field outside the visible sequence
fields, or when `n_in_sequence` exceeds the total number of fields; such
descriptors previously built a malformed type. The failure is propagated by
`PyStructSequence_NewType()`, `PyStructSequence_InitType()` and
`PyStructSequence_InitType2()`.

Add `_testcapi` helpers and regression tests for both rejected cases, and
document the invariants in the C API reference.

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count_members() now rejects a PyStructSequence_Desc whose n_in_sequence is
negative, completing the bounds check (only n_in_sequence <= n_fields was
enforced before). A negative value otherwise inflated tp_basicsize and failed
later with a cryptic SystemError from PyTuple_New().

Also fix the two existing error messages: n_in_sequence is an int but was
printed with %zd (Py_ssize_t), which is undefined behavior in a variadic call.

Add a _testcapi helper and regression test for the negative case, and document
the non-negative constraint.

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