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Dereferencing `ST(0)` inside the XSUB is safe: the typemap's INPUT block has already
croaked unless `SvROK(sv) && SvMAGICAL(SvRV(sv))`, and OUTPUT overwrites `ST(0)`
only afterwards.

## When the C library frees your children behind your back

Some teardown calls free objects the Perl side still holds pointers to —
`foolib_close()` frees every handle the connection owns. A NULL check cannot see it:
the pointer is unchanged and now points at freed memory.

The fix is a **generation counter**. The parent counts the events that invalidate
children; each child stores the count it was born under:

```c
/* parent, before the invalidating call */
self->generation++;
foolib_close(self->conn);

/* child, before touching its handle */
static int foo_conn_stale(pTHX_ SV *conn_sv, unsigned int generation) {
    MAGIC *mg = conn_sv && SvMAGICAL(conn_sv)