vendor(apptype): emit TransportProtocol.tcp instead of TransportProtocol.get(...) in 23,938 places #768

Is your feature request related to a problem? Please describe.

The generated registries call TransportProtocol.get('tcp') to name a constant. There are 23,938
such calls across pcapkit/const/reg/apptype/, every one executed at class-definition time, i.e. on
every import of pcapkit. get() is a real function doing an isinstance check, key.lower()
twice, a __members__ membership test and a __getitem__ — to reach a member that plain attribute
access returns in one lookup.

Describe the solution you'd like

Emit TransportProtocol.tcp instead of TransportProtocol.get('tcp'). One line, in
pcapkit/vendor/reg/apptype/apptype.py:573-574:

return ' | '.join(f'TransportProtocol.get({proto!r})' for proto in sorted(protos, key=order.index))

Measured, on this checkout:

T.get('tcp') is T.tcp  ->  True        (same for udp / sctp / dccp / undefined)
per call:   get() 524 ns   attribute 24 ns   ->  21.4x
23,938 sites:  12.5 ms  ->  0.6 ms    ~12 ms saved per import
generated size: ~187 KB smaller (29 chars -> 21 per site)

Safe because the generator only ever emits the five declared names (TRANSPORTS + ('undefined',)),
so get()'s extend_enum fallback for an unknown name cannot fire in generated code. get() itself
stays — AppType.get(..., proto=...) and other callers need it.

Describe alternatives you've considered

Leaving it: the cost is small in absolute terms. But it is pure waste on every import, and the
explicit form also reads better — cls.__transport__ is TransportProtocol.tcp says plainly that it
is an identity comparison against a canonical member, which is the thing #732 wants to make safe.

Additional context

Note the members are lowercase: TransportProtocol.TCP raises AttributeError, since only
get() lowercases. Requested by the maintainer. Should land together with #744's one-line change to
:717 in the same file — both are mechanical and one regeneration covers both, where separate PRs
would produce conflicting ~12,000-line diffs over the same generated files.