Write a superseding ADR: 0002 says two inter-VLAN rules, there are three

[Gerrrt]

ADR-0002:40
states:

Exactly two inter-VLAN rules exist.

There are three. docs/security.md:44-49 admits it in prose, and puts it well:

an undocumented rule is still an undocumented rule.

ADR-0001
requires a superseding ADR rather than an edit to the original — the record
of what was decided and when is the point, and quietly changing "two" to "three"
destroys exactly the thing the ADR process exists to preserve.

So this needs ADR-00NN, "Supersedes 0002", stating:

• what the third rule is, in which direction, and why it was added;
• when it was added, and why it was not recorded at the time;
• the current count and the full list.

Do it before, not after

ADR-0008 adds two more rules (50→40 and 99→20), taking the estate to five. If
that lands first, the superseding ADR has to reconstruct two separate pieces of
history instead of one, and the README's claim that "three inter-VLAN rules
exist, each directional and documented" will be wrong by two rather than by
nothing.

The README (README.md:41-42) currently says three, so the README and ADR-0002
already disagree with each other.