Hoist protocol dispatch into module-level tables

[EONRaider]

Target: 1.4.0 (Tier 1 — earn the benchmark)

_ethertype_class() (layer2/ethernet.py:21) and
_ip_protocol_class() (layer3/ip.py:48) both run their deferred
imports and rebuild a dict literal on every single call. They are
called once per layer per frame, so this is the hottest path in the
library.

Profiling a corpus walk puts _ip_protocol_class at the top by
tottime, with _ethertype_class third.

Measured

Against a module-level dict built once:

Function	Speedup
_ip_protocol_class	91×
_ethertype_class	42×

What to do

Build each table once at import (lazily on first use, to preserve the
existing import-cycle avoidance) and reduce the call to a dict lookup.
The ipv6= gating of _IPV6_ONLY_NUMBERS must be preserved — an IPv4
packet with protocol=0 must still not decode a Hop-by-Hop layer.

This is the same change as the public registry in #87 (Tier 2).
Doing it as a plain table now and generalising it into the registry
later is fine, but the two should not be designed in isolation.

Acceptance criteria

• Dispatch is a lookup, not a construction, on the hot path.
• The IPv6-only gating behaviour is unchanged and still tested.
• Full suite green.

Part of #103.