Align the layer sub-package exports with the top-level package

[EONRaider]

Target: 1.3.1 (Tier 0 — hygiene)

The per-layer __init__.py files export a subset of what the layer
actually ships, while the top-level netprotocols/__init__.py exports
everything. Importing by layer therefore fails for classes that plainly
belong to it:

• layer7/__init__.py exports only DNS — missing DHCP,
  DNSOverTCP and DNSResourceRecord.
• layer3/__init__.py omits GRE, and the option value types
  IPv4Option, IPv6Option, NDPOption and IGMPv3GroupRecord.

from netprotocols.layer7 import DHCP fails today; from netprotocols import DHCP works. That asymmetry is surprising and undocumented.

What to do

Bring each layer's __init__.py in line with what the layer's modules
define and the top-level package re-exports, keeping __all__ sorted
and consistent with the existing style.

Acceptance criteria

• Every public class importable from netprotocols is also
  importable from its own layer sub-package.
• A test asserts the two export sets agree, so this cannot drift
  again.