fix(misc): IP_Pseudo misses a malformed IPv6 extension header past the first one #2513

On main, IP_Pseudo (pcapkit/protocols/misc/checksum/ip_pseudo.py, from #2479) misses a malformed IPv6 extension header that sits past the first one. The original #2479 worker raised this in its own close-out; I checked it against the code.

• _source / _destination: the guard for "a malformed Destination Options / Routing header stopped the walk" only tests ip.info.next, the first header after the fixed header. With a valid Hop-by-Hop header in front of a malformed Destination Options (or Routing) header, the guard does not fire, and the address silently falls back to the fixed header's. A Home Address option, or the route's final destination, is then lost from the pseudo-header without any error.
• _upper: when the walk ends on Raw, the error says "give src" / "give dst". Supplying those does not help derive the protocol or the length, which is what _upper needs.
• test_ip_pseudo_2471_unit.py covers only the first-header cases.

Fix: detect the header the walk stopped on at any depth, in the same way _upper already walks extension_headers, word _upper's error for what it actually needs, and add tests that fail without the fix.

Blocked: the checksum wave (#2470, #2472–#2475) is editing pcapkit/protocols/misc/checksum/* right now. This goes onto that branch once its worker hands back, rather than as a parallel edit that would collide.