docs(architecture): import the FDS architecture decisions from the lab repo (closes #159) (#160)

## What & why

Brings the engineer-facing FDS documentation next to the code it describes. Imported from `projects/open-register-fd/` in `Respellion/innovation-lab` and translated to Dutch: **six ADRs**, the ADR index and template, the **L3 component view**, and the **slice-1 proposal**.

The architecture blueprint, the FDS gap analysis and the two privacy views stay in the lab repo — the OKRs cite them and they feed tender responses. Each side names the split in a "Wat ligt waar" table, so nothing is documented twice.

Closes #159

### Why `docs/architecture/fds/` and not `docs/architecture/`

This repo's own ADR series now runs `adr-0001-loose-coupling` … `adr-0010-bff-oidc`. The imported set is numbered 0001–0006, so a flat import would collide across the whole imported range. The subfolder preserves the imported numbering, and with it roughly thirty `ADR-000N` cross-references inside the imported text that would otherwise all need rewriting.

In the MkDocs sidebar the imported six appear as **FDS ADR-000N** so they are not confused with this repo's series. `docs/architecture/fds/README.md` explains the two series.

### Mermaid support was missing

`pymdownx.superfences` had no `custom_fences`, so the imported diagrams would have published to Gitea Pages as raw code blocks. This PR adds the mermaid custom fence, the nav group, and one link under *Where to go* in the docs index.

## Definition of Done

- [x] Linked Gitea issue (above).
- [ ] Failing test committed before the implementation. — n/a, documentation only.
- [ ] Implementation makes the test pass. — n/a, documentation only.
- [x] Conventional Commits referencing the issue (`refs #159`).
- [x] Rebased on current `main`; no conflicts.
- [ ] CI green — n/a for content; the docs verification is below.
- [ ] `docker compose up` reaches green health checks. — n/a, no runtime change.
- [x] Docs updated if behaviour, contracts, or operations changed.
- [x] ADR added in `docs/architecture/` if a non-obvious decision was made. — six imported, plus the numbering decision recorded in the folder README.
- [ ] Demo note in `docs/demo-script.md`. — n/a, nothing user-visible.

## Verification run

- `mkdocs build` — clean. No missing-nav warning for any `architecture/fds/` entry. The two remaining warnings are pre-existing on `main` and untouched here: the set of pages absent from `nav`, and a broken link in `runbooks/ci.md` to `services/acl/stryker-config.json`.
- Mermaid renders as a diagram, not a code block: `site/architecture/fds/c4-component-view/index.html` contains `class="mermaid"`.
- All relative markdown links in the repo resolve.

## Notes for reviewers

- **Language.** The imported documents are Dutch; this repo's own documents remain English. Deliberate, not an oversight — the lab repo standardised on Dutch and these pages moved with it. Translating the rest is a separate decision.
- **Ownership.** This repo sits in the `eho/` namespace while it now holds the canonical FDS architecture decisions that tender answers point at. Worth deciding whether it should move to `Respellion/`.
- **Scope drift, not fixed here.** The imported text is faithful to its source, so the slice-1 proposal and the ADRs assume NHR/KVK for slice 1, while the lab-side blueprint still uses BAG as its example register. The lab-side documents carry a banner about this; Blueprint v2 (slice 5) is where the diagrams get corrected.
- **Companion PR:** `Respellion/innovation-lab` #34 holds the lab-side half of this split.Reviewed-on: #160

docs/architecture/fds/adr/0001-acl-at-every-register-boundary.md

@@ -0,0 +1,42 @@
+# ADR-0001: Anti-Corruption Layer op elke registergrens
+
+- **Status:** accepted
+- **Datum:** 2026-06-13
+- **Deciders:** Lab Circle (Build, Lead Link)
+- **Vervangt / vervangen door:** —
+
+## Context
+
+De applicatie bevraagt meerdere registers: BRP, NHR/KVK, en ZGW via OpenZaak. Hun vocabulaires en
+schema's verschillen van elkaar en van ons domein. Zij veranderen ook zelf mee met de FDS-standaarden.
+
+Lekt registervocabulaire het domeinmodel in, dan werkt elke wijziging aan de registerzijde door in de
+bedrijfslogica. Het domein wordt dan een lappendeken van vreemde begrippen in plaats van ubiquitous
+language.
+
+## Besluit
+
+Elk register is bereikbaar via een Anti-Corruption Layer: **één adapter per register**, die een
+**port** vervult die het domein definieert.
+
+Adapters doen alleen vertalen en velden versmallen. Zij bevatten geen bedrijfslogica. Het domein
+spreekt `Persoon` en `Organisatie`, en nooit veldnamen uit BRP of NHR.
+
+## Gevolgen
+
+**Positief:** verloop in registers en FDS-standaarden blijft bij de adapter. Het domein blijft stabiel
+en testbaar. Adapters zijn onafhankelijk vervangbaar, en dat is precies wat de FSC-wissel uit
+ADR-0002 goedkoop maakt. Het patroon generaliseert naar een herbruikbare ACL-template per register,
+een Foundations-kandidaat.
+
+**Negatief en kosten:** één vertaalmap per register om te schrijven en te onderhouden, plus een extra
+indirectie die engineers moeten respecteren in plaats van omzeilen.
+
+**Vervolgwerk:** extraheer de ACL-template zodra de tweede adapter bestaat (slice 3).
+
+## Overwogen alternatieven
+
+- **Registers direct aanroepen uit de applicatieservices** — afgewezen: dit koppelt bedrijfscode aan
+  registerschema's en aan versies van FDS-standaarden.
+- **Eén generieke registeradapter** — afgewezen: registers verschillen genoeg dat een generieke
+  abstractie zou gaan lekken of opzwellen. Adapters per register zijn duidelijker.