docs(registry): capability-conformance plugin note for #159

Description

The pitch.

## Problem

#159 (capability-conformance registry gate, merged) shipped the
`verified_capabilities` plugin, validator, and scenario without a
`docs/layers/registry.md` entry, unlike #148's `byzantine_gossip` section.
The merged commit also carried a `Co-Authored-By: Claude Sonnet 5` footer,
per reviewer feedback that should not recur in follow-up commits.

## Change

- Adds a `## Capability-conformance plugin: verified_capabilities`
  subsection to `docs/layers/registry.md`, mirroring #148's
  `byzantine_gossip` section: threat model, mechanism (bootstrap allowance,
  per-capability exclusion, re-admission), validator description, and demo
  scenario with the literal FAIL/PASS output and the exact test commands.
- No code changes; no other doc sections touched.

## Test

`make ci-local`: 1282 passed, 1 skipped, 1 deselected, all 5 checks green.

## Fit

Same documentation pattern #148 already established for a hardened registry
plugin variant, applied to `verified_capabilities`.

## Note

Includes a one-line lint fix for a pre-existing ruff failure on main
introduced by #184 (test_negotiation_determinism.py:88), isolated in its
own commit so the docs change stays clean.