GeezerVison

Fixes #733.

Packer.pack() checks for existing buffer exports before it starts, but nothing stopped a default() callback from calling getbuffer() partway through the same call and then having the packer grow its buffer to fit the rest of the object. msgpack_pack_write() reallocates through PyMem_Realloc without checking whether anything holds a live view onto the old allocation, so growing the buffer mid-pack can move it out from under an export that Python still considers valid. Under ASan this shows up as a straightforward heap-use-after-free the moment anything reads through the export afterward, matching what's reported in the issue.

The exports counter used to live only on the Cython Packer object, so the plain C write path in pack.h had no way to see it. I moved it into the msgpack_packer struct itself and made msgpack_pack_write raise BufferError instead of reallocating whenever exports is nonzero and growth is needed - the same error _check_exports() already raises for every other buffer-mutating method, just reachable from the one path that runs mid-pack rather than at the start of a call.

Added a regression test (buf_size set small enough that the default() return value forces a realloc) confirming it raises BufferError with the fix and silently proceeds without it. Full suite passes locally (139 passed, 1 pre-existing skip).

I wasn't able to get an ASan-instrumented build running in the time I had, so I haven't reproduced the crash itself the way the issue's reporter did - the fix is based on reading the allocation/export lifecycle directly rather than on catching it under a sanitizer, and the raised BufferError is verified to actually happen with git stash on both sides of the change.