pcapng: journal entry split on b'\n\n' shreds binary data that contains it #723

Pre-existing on main, confirmed identical on #722's branch (not introduced or fixed there).

A: a binary field's value containing \n\n splits the entry mid-value.
b'MESSAGE\n' + pack('<Q',5) + b'a\n\nb\n' -> MESSAGE=b'a' plus a junk entry named b declaring a bogus length, 2 warnings. Same shape on both trees (byte counts differ by 1 on #722's branch, an artifact of its terminator-newline restore, not a new defect).

B: a binary field's own 8-octet length prefix can equal \n\n + 6 NULs, since:

>>> struct.pack('<Q', 2570)
b'\n\n\x00\x00\x00\x00\x00\x00'

So a 2570-octet binary field splits mid-prefix and the entry parses zero fields. Identical on main and #722: data=[{}, {}], 1 warning.

Root cause: self.entry.split(b'\n\n') in SystemdJournalExportBlock.post_process (pcapkit/protocols/schema/misc/pcapng.py) can't tell a real entry separator from \n\n inside binary data.

Refs: #704, #722.