fix(pcapng): out-of-range if_tzone and if_tsoffset raise raw ValueError, OSError and OverflowError #1340

Found while cross-reviewing #1331, which makes these reachable for the first time by decoding signed fields as signed. Out of scope there.

PCAP-NG if_tzone and if_tsoffset are now decoded as signed, so an out-of-range value reaches datetime rather than being masked into a large positive one. Three raw exceptions escape:

• if_tzone of ±24h or more raises a bare ValueError from timezone(timedelta(seconds=...)).
• if_tsoffset out of range raises OSError or OverflowError from datetime.fromtimestamp, which the surrounding code catches only ValueError from.
• if_tsoffset = -2**63 shifts from OverflowError on main to OSError after fix(corekit): signed NumberField decode, zero-length PayloadField, PCAP trailing bytes and empty Raw #1331.

All three should be in-library errors from pcapkit.utilities.exceptions, so a malformed capture reports as a protocol error rather than as an interpreter traceback from datetime.

Blocked on #1331, which owns the signed-decode change.