refactor(corekit)!: move utilities.decorators into corekit, and state the corekit/utilities rule (#1712, option A) #1735

Describe the bug
Under the placement rule chosen on #1712 (option A), utilities holds cross-cutting services that import nothing from pcapkit outside utilities, and corekit holds the types the packet model is built from. pcapkit/utilities/decorators.py breaks that rule:

• seekset, beholder and prepare act on Protocol._file and _seekset;
• beholder lazily imports protocols.misc.null and protocols.misc.raw (decorators.py:130-132).

Expected behavior

• The move: decorators.py moves to pcapkit/corekit/decorators.py, with no re-export left behind (the ruling on refactor(corekit)!: move the byte-order field helpers into corekit (#1711) #1715). Every importer updates its path: protocols/protocol.py, internet/internet.py, internet/ipv6.py, schema/schema.py, plus the tests and docs.
• What breaks: pcapkit.utilities.{seekset,beholder,prepare} and the logger name pcapkit.utilities.decorators. Subject refactor(corekit)!:, labelled breaking.
• No new import cycle: the module's top-level imports are only utilities.exceptions and utilities.logging. Do not re-export it from utilities/__init__.
• Write the rule down: docs/source/contributing/conventions/ gains the rule, with a project test enforcing it: no module under pcapkit/utilities/ imports from pcapkit outside utilities, whether at top level, inside a function or under TYPE_CHECKING.

Blocked until #1730 merges. It also edits protocol.py's import lines.