docs(contributing): record protocol member order convention

Group members as properties, methods, data models, utilities, per the owner ruling.
Reference: IPv4, TCP, UDP, ARP and Ethernet.

Author: JarryShaw

docs/source/contributing/conventions/index.rst

@@ -17,6 +17,7 @@ House Conventions
    * :ref:`extension-header-subclassing` -- a protocol class hierarchy;
    * :ref:`protocol-family-grouping` -- how a protocol family's base and members
      spell :meth:`~pcapkit.protocols.protocol.Protocol.id`;
+   * :ref:`protocol-member-order` -- the order of members in a protocol class body;
    * :ref:`protocol-layer-placement` -- which subpackage a dissector belongs in;
    * :ref:`process` -- the repository rather than any of its code;
    * :ref:`documentation` -- the prose itself, on these pages and in the API
@@ -41,6 +42,7 @@ House Conventions
    registry-protocol
    extension-header-subclassing
    protocol-family-grouping
+   protocol-member-order
    protocol-layer-placement
    process
    documentation

docs/source/contributing/conventions/protocol-member-order.rst

@@ -0,0 +1,52 @@
+.. _protocol-member-order:
+
+Protocol Member Order
+---------------------
+
+The order of members inside a protocol class body. Settled by the owner as a
+ruling, so a new dissector and a refactor land in the same layout.
+
+Each group sits under its section comment, spelled as the repo spells it:
+``# Properties.``, ``# Methods.``, ``# Data models.`` and ``# Utilities.``
+
+The Order
+~~~~~~~~~
+
+#. **Properties.** Every ``@property`` member.
+#. **Methods.** Public methods: mostly ``id()``, ``read()``, ``make()`` and
+   ``register_xxx()``.
+#. **Data models.** Every dunder name: ``__index__``, ``__length_hint__``,
+   ``__init__``, ``__new__``, ``__post_init__`` and any other.
+#. **Utilities.** Everything else, mostly private methods. A ``_read_xxx()`` comes
+   before its ``_make_xxx()``, and any such pair keeps that order.
+
+Example
+~~~~~~~
+
+.. code-block:: python
+
+   class Foo(Protocol[Data_Foo, Schema_Foo]):
+
+       # Methods.
+       def read(self, length, *, ...):
+           ...
+
+       def make(self, ...):
+           ...
+
+       # Utilities.
+       def _read_foo_field(self, ...):
+           ...
+
+       def _make_foo_field(self, ...):
+           ...
+
+The Reference
+~~~~~~~~~~~~~
+
+:class:`~pcapkit.protocols.internet.ipv4.IPv4`,
+:class:`~pcapkit.protocols.transport.tcp.TCP`,
+:class:`~pcapkit.protocols.transport.udp.UDP`,
+:class:`~pcapkit.protocols.link.arp.ARP` and
+:class:`~pcapkit.protocols.link.ethernet.Ethernet` follow this order. When a
+new class is unclear, match them.