fix(toolkit): pyshark's fallback ends the extraction when tshark reports no ip.dst, and keys a source-routed segment by the route's last hop #1671

Describe the bug
With engine='pyshark', _from_layers (pcapkit/toolkit/pyshark.py:601-625 at #1670's head 6890f38f6) keys a TCP segment by tshark's ip.dst, without a guard (:622). Two consequences:

• The extraction ends. When tshark reports no ip.dst, a bare AttributeError escapes extract(). Measured on main (6c17c7e1b), 3.11 with tshark 4.6.9: TCP behind an LSRR or SSRR option of length 9, where tshark reports "Suboption would go past end of option". 12 such captures raised.
• Wrong destination. While an LSRR or SSRR route is still live, tshark moves ip.dst to the route's last address, so a source-routed segment is keyed by the last hop. The default engine keys it by the header's destination.

#1670 avoids both whenever it reads the frame's octets (_network, :827-836). _from_layers is still the fallback where the frame cannot be had (:955-963), and there both remain.

Expected behavior
The fallback never raises on a missing field, and it keys a source-routed segment by the header's destination (tshark's ip.cur_rt, or the octets at ip.version.pos), as the default engine does.

Blocked until #1670 merges, because it rewrites this file. The pyshark part of #1609 waits on the same PR.