test(project): guard protocol class member order (#2490)

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

@@ -3,8 +3,8 @@
 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.
+The order of members inside a protocol class body. Settled by the owner on
+:issue:`2490`, 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.``
@@ -17,7 +17,7 @@ The Order
    ``register_xxx()``.
 #. **Data models.** Every dunder name: ``__index__``, ``__length_hint__``,
    ``__init__``, ``__new__``, ``__post_init__`` and any other.
-#. **Utilities.** Everything else, mostly private methods, in three subgroups. Every private helper, including ``_check_*``, sits here, never before ``_make_data()``.
+#. **Utilities.** Everything else, mostly private methods, in the subgroups below. Every private helper, including ``_check_*``, sits here, never before ``_make_data()``.
 
    a. **Data.** ``_make_data()`` comes first in the group, before everything else.
    b. **Helpers.** The helpers that ``read()`` or ``make()`` call, one helper at a
@@ -26,17 +26,33 @@ The Order
       through ``getattr``, or through the registry maps (``__proto__``,
       ``__option__``, ``__parameter__`` and similar), do not count. They are ordered
       by the enum value order of the corresponding code, or by the registry's order
-      where there is no enum. Each helper's
+      where there is no enum. A registry fallback that ``read()`` or ``make()``
+      names directly, such as ``_read_msg_unknown()`` in
+      :class:`~pcapkit.protocols.internet.mh.MH`, does not count either: it is a
+      specific reader, ruled on :issue:`2490`. Each helper's
       ``_read_`` and ``_make_`` methods stay together, with the reader first:
       ``_read_mac_addr()`` then ``_make_mac_addr()``.
    c. **Specific readers and makers.** The option, type and parameter methods,
-      all ``_read_*()`` before all ``_make_*()``. Each set is ordered by the enum
-      value of the code or type it handles, not alphabetically by name.
+      all ``_read_*()`` before all ``_make_*()``. Each set opens with the registry's
+      default, its fallback for a code with no entry of its own, as
+      ``_read_opt_unassigned()`` opens the readers of
+      :class:`~pcapkit.protocols.internet.ipv4.IPv4`; the rest follow the enum value
+      of the code or type each handles, not alphabetical order. Ruled on
+      :issue:`2490`.
+   d. **Other helpers.** Every helper that ``read()``, ``make()`` and the registries
+      do not reach goes last, including one that only another helper calls, such as
+      ``_decode_text()`` in :class:`~pcapkit.protocols.application.httpv1.HTTP`.
+      Ruled on :issue:`2490`.
 
 **Registry maps.** The registry maps themselves (``__proto__``, ``__option__``,
 ``__parameter__`` and similar) are ordered by their enum values, or alphabetically
 by key where there is no enum.
 
+**The guard.** :file:`tests/project/test_protocol_member_order_2490_unit.py` holds
+every protocol class to the groups, ``_make_data()`` first, each ``_read_xxx()``
+before its ``_make_xxx()``, and subgroup b, with a table of known exceptions
+(:issue:`2490`).
+
 Example
 ~~~~~~~