fix(const): pcapkit.const does not re-export the Linux ARPHRD registry #2661

pcapkit/const/__init__.py re-exports every const subpackage except linux. It has no from pcapkit.const.linux import *, so ARPHRD is missing from pcapkit.const.__all__. #2656's new guard found this, and its test lists linux as a known gap that cites this issue.

The subpackage also breaks the house naming: pcapkit.const.linux.__all__ lists ARPHRD. Every other package re-exports its registries under a package prefix, as pcapkit.const.arp does with ARP_Hardware, and pcapkit.vendor.linux already exports Linux_ARPHRD. The registry came in with #2443 and #2497, and neither is in a release (tags stop at v1.5.1), so renaming it breaks nothing shipped.

Fix:

• pcapkit.const.linux re-exports the class as Linux_ARPHRD.
• pcapkit/const/__init__.py star-imports it.
• The docs index lists it.
• The KNOWN_UNWIRED entry in test_const_package_exports_2656_unit comes out, as that test then requires.
• Callers that import the name from the package follow it. Callers that import from pcapkit.const.linux.arphrd are unaffected.