fix(protocols): export Protocol from pcapkit.protocols.protocol (#1770)

[JarryShaw]

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Summary

pcapkit/protocols/protocol.py's __all__ listed only ProtocolBase, so from pcapkit.protocols.protocol import * did not bind Protocol, the module's user-facing class. It now lists both. ProtocolMeta stays out, as protocol.rst files it under internal definitions.

• Unchanged on purpose: the register error still names Protocol while the guard tests ProtocolBase. fix(foundation): registrar error messages and type hints name the public class, not the Base the guard checks #1016 settled that the public door names the public class.
• Test: tests/protocols/test_protocol_module_exports_1770_unit.py pins __all__, the star-import, and the guard's width. With the __all__ change reverted, 2 tests fail.
• Runs: the new file and its neighbours: 57 tests OK. tests/project: 930 passed, 1 skipped.

Closes #1770

[JarryShaw]

Verdict: GOOD TO GO at 777f000a7. A Sonnet cross-review (author: Opus), after one fix round. The first review caught that the error-message half reversed #1016, so that half was dropped. Re-checked: only the __all__ line changes in protocol.py, the guard pins match main, the test bites, and tests/project passed 930. The branch is behind main but merges cleanly.