feat(engines): type the third-party engine packages with local stubs and a per-engine _expkg type #1878

The engines' handle on their third-party package, _expkg, is ModuleType after #1870, so mypy sees every call through it as Any. Per the owner's request on #1870, give each engine real typing.

Current state (mypy 2.3.1, the repo's mypy.ini with ignore_missing_imports = True):

• scapy ships py.typed.
• dpkt and pyshark are untyped.
• pcap (pypcap and pcap-ct) and pcapfile are untyped and are not in the test or docs environments.

Proposed:

1. Local .pyi stubs, covering only the APIs and data types pcapkit uses, for dpkt, pyshark, pcap and pcapfile. Put them under one stub directory registered via mypy_path, and keep them out of the wheel and sdist.
2. A per-engine type for _expkg. mypy rejects a module as a type, so use a Protocol per engine, in a TYPE_CHECKING block, describing the attributes the engine reads. The runtime stays lazy-import as it is.
3. mypy on pcapkit/foundation/engines/ and pcapkit/toolkit/ must report no new errors, and calls through _expkg must type-check against the stubs. A test should pin that each engine's _expkg annotation is its own type and not ModuleType.

Builds on #1870, which removed the class-level engine imports.