Migrate IPv4 and HIP option dispatch from setattr/getattr to the registry pattern #429

IPv4 and HIP dispatch their option/parameter handlers by building a method name and
getattr-ing it off the instance, while the other fourteen dispatch families go
through a class-level registry dict. Two idioms for one job.

The two forms

IPv4 / HIP — register_option installs the handler as a class attribute:

setattr(cls, f'_read_opt_{name}', meth[0])          # ipv4.py:458
setattr(cls, f'_read_param_{name}', meth[0])        # hip.py:590

and the read side derives the same name from the enum member:

meth_name = f'_read_opt_{name}'
meth = getattr(self, meth_name, self._read_opt_unassigned)

Everything else — a defaultdict keyed by code, holding a method-name string or a
(parser, constructor) pair:

name = self.__option__[kind]
meth = getattr(self, f'_read_mode_{name}', self._read_mode_donone)

Why migrating is worth it

• Uniformity. 14 of 16 families use the registry; these two are the outliers, so
  anyone reading the codebase has to learn both.
• Inspectability. len(TCP.__option__) is 22 and readable as data. IPv4's 15
  handlers can only be found by probing dir() for _read_opt_*, so "what is
  registered?" has no direct answer.
• Registration is irreversible and namespace-global. setattr permanently installs
  a method on the class with no way to enumerate or undo it. The overwrite warning
  keys on hasattr(cls, f'_read_opt_{name}'), which is true for every shipped
  handler too, so it cannot distinguish a user registration from a built-in.

What it is not

Not a defect fix. I checked the hazard that would have made it one: across both enums
(IPv4 30 members, HIP 62) no member name collides with the
_read_opt_unassigned / _read_param_unassigned fallback, and none produces an
invalid identifier. The setattr form is not currently unsafe — it is just the less
legible of the two.

Worth noting the irony: the setattr form never had the lookup-miss leak that #425
fixed for the registry form, precisely because it has no dict to inject into.

Sequencing

After #427 and #428 merge. #428 reworks the ProtocolBase._lookup_registry helper a
migration would build on, and register_ipv4_option / register_hip_parameter are
public API — both signatures have to keep working while what happens underneath
changes, so the tests should pin the public behaviour before the internals move.