refactor(interface): make the registry-entry helpers public in pcapkit.interface.registry (#1519)

[JarryShaw]

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• Move: _same_entry, _restore_shipped and _restore_shipped_dumper leave foundation/extraction.py for the new pcapkit/interface/registry.py. Their public names are same_entry, restore_shipped and restore_shipped_dumper, and the bodies are unchanged. I named the module after what the helpers operate on, keeping the one-word style of core and misc.
• Callers: extraction.py imports the helpers at top level, and its eight call sites use the public names. The lazy import in TraceFlowBase.register_dumper now points at the new module and stays inside the function. It no longer needs to: hoisting it to top level imports cleanly from every entry point.
• Public surface: the helpers are not re-exported from pcapkit.interface or pcapkit. They are registrar plumbing, and pcapkit.interface.misc is not re-exported either. pcapkit.__all__ is unchanged.
• Docs: there is a new interface/registry.rst page, listed in the interface toctree and table.
• Tests: tests/interface/test_registry.py has 15 tests and all 15 fail on origin/main (module missing). Five mutations of the helpers, such as an asymmetric same_entry, a dropped silent no-op, or a subscript lookup on a defaultdict, are each caught. Coverage of registry.py is 100% of lines and branches. With the traceflow.py edit reverted, 2 tests in test_restore_builtin_unit.py fail with ImportError.

Closes #1519

[JarryShaw]

Verdict: GOOD TO GO at 693702786. Cross-reviewed on Sonnet; the author ran Opus.

• Behaviour is unchanged: each moved function's AST is identical to main's, apart from the renames and two added docstring sentences. No old underscore name is left anywhere in the tree. All 8 call sites and the lazy import in traceflow.py:246 point to the new module.
• No import cycle: 12 entry points each import first in a fresh interpreter. pcapkit.__all__ (64 names) and dir(pcapkit) (93) match main.
• Docs: a full nitpicky Sphinx build shows no new warnings, and every link on the new page resolves.
• Fail-before: tests/interface/test_registry.py gives 15 failed on main. test_restore_builtin_unit.py gives 2 failed with main's traceflow.py.
• Tests: these pass: interface (35), registry (43), extraction (36), traceflow (45), public API and isolation (16), and the project guards (130+).
• Closes #1519: every inventory row is accounted for: refactor(schema): share the IPv6-option and byte-order schema helpers (#1519) #1528, refactor(http): move test_start_line and its patterns into the http base module (#1519) #1525, the packet2chain ruling, and this PR.

Non-blocking:

• Test gap: 6 of 7 mutations are caught. The survivor treats two distinct classes with the same __name__ as the same entry, so the docstring's "same object" claim isn't pinned.
• Untracked private import: fix(toolkit): trace and reassemble a TCP segment whose header the snapshot cut (#1518) #1556 added pcapkit/toolkit/pcapng.py:21, which imports the private _tcp_segment from toolkit/pcap.py. That is the pattern refactor: move shared helper functions into utilities or corekit by use case #1519 targets, and nothing tracks it. I'm filing it as its own issue.