docs: drop dangling FOCAS refs + link unreferenced v2 design docs

- docs/drivers/FOCAS.md and docs/v2/implementation/focas-wire-protocol.md
  pointed at focas-deployment.md and focas-simulator-plan.md, both of
  which were untracked drafts that have since been removed. Drop the
  refs (the wire-protocol companion now stands on its own; deployment
  guidance lives inline in the FOCAS driver doc).
- Link the orphan v2 design docs from docs/README.md (multi-host
  dispatch, v2 release readiness, the historical lmx-followups tracker)
  and from modbus-test-plan.md (s7.md, mitsubishi.md per-family quirk
  catalogs, sibling to dl205.md).

Surfaced by the doc audit; no content changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/v2/modbus-test-plan.md

@@ -70,6 +70,17 @@ integration tests until reproduced on hardware:
 - TxId drop under load (forum rumour; not reproduced).
 - Pre-2004 firmware ABCD word order (every shipped DL205/DL260 since 2004 is CDAB).
 
+### Siemens SIMATIC S7
+
+Quirk catalog at [`s7.md`](s7.md) — covers S7-1200 / S7-1500 / S7-300 / S7-400 /
+ET 200SP. Modbus TCP isn't native; each platform exposes it via a different
+add-on module with its own register-mapping conventions.
+
+### Mitsubishi MELSEC
+
+Quirk catalog at [`mitsubishi.md`](mitsubishi.md) — Modbus TCP via add-on modules
+across the MELSEC family.
+
 ### Future devices
 
 One section per device class, same shape as DL205. Quirks that apply across