docs: reference guides for stamdata, audit log, feature flags + document-feature skill

Three how-it-works/how-to-extend reference docs (docs/reference/), each with a
coupling section, indexed in docs/README.md. New document-feature skill so docs
ship in the same diff as the code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/reference/stamdata.md

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+# Stamdata (config-as-code reference data) — how it's built & extended
+
+Business-tunable reference data — the tables the business controls (profession↔diploma map,
+policy-question text, thresholds) — is **typed, checked-in config changed via git → PR →
+build**, not a runtime-editable database. For the _why_, see
+[ADR-0004 — Stamdata as code](architecture/0004-stamdata-as-code.md); this page is _how the
+code is laid out and how to add a table without coupling_. Built in WP-29, hardened in WP-48.
+
+## The one rule that shapes everything: no runtime write path
+
+The catalog is the source of truth and lives in code. The admin editor **downloads** an
+edited `{table}.json` for a human to commit — it never PUTs. The authority that a change is
+valid is the **CI gate**, not the server. This is what keeps a bad edit out of prod instead
+of out of the database.
+
+## Layered pipeline
+
+Backend (`backend/src/BigRegister.Api/Stamdata/`):
+
+- `StamdataCatalog.cs` — the registry. Adding a table is **one line**:
+  `StamdataTable.Of<ProfessionMapping>("professions", …)`.
+- `StamdataTable.cs` — generic table model: columns are _reflected_ from the typed record
+  (first property = key; temporal iff it has `geldigVan`+`geldigTot`). Holds `Validate()`
+  (referential integrity) and `RowsOn(date)`.
+- `StamdataFile.cs` — reads the JSON as an **embedded resource** (identical read from API
+  and test assembly). No write method exists.
+- Endpoints in `Program.cs`: `GET /stamdata` and `GET /stamdata/{table}?peildatum=` —
+  **reads only**, both behind the `StamdataAdmin` gate.
+
+Frontend (`src/app/beheer/`, the `beheer` context):
+
+- `domain/stamdata.ts` (pure model + `activeOn`/`rowErrors`/`toJson`),
+  `domain/stamdata-editor.machine.ts` (Elm-style union — **no save Msg by design**).
+- `contracts/stamdata.dto.ts` → `infrastructure/stamdata.adapter.ts` (`list()`/`load()`
+  with a `parseStamdataTable` trust boundary; **no write method**).
+- `application/stamdata.store.ts` — root singleton; derives `dirty`/`counts`/`errors`;
+  `download()` serializes the edited table to a blob to commit.
+- `ui/stamdata.page.ts` (thin container) + `ui/stamdata-table-editor/` (the generic grid).
+
+## How to add a table (zero UI code)
+
+1. Add a typed record + its `professions.json`-style embedded JSON in `Stamdata/`.
+2. Register it with one line in `StamdataCatalog.cs`.
+3. If it references another table, add a `StamdataRef` to the CI gate (below).
+
+That's it — the editor grid renders from the reflected column schema, so **no per-table
+component**. This is the payoff of the schema-driven design.
+
+## The CI gate is the authority
+
+`backend/tests/BigRegister.Tests/StamdataValidationTests.cs`. `Every_catalog_table_is_valid`
+covers every registered table generically; the `StamdataRef` list catches dangling
+references (today: `Diploma.Opleiding → professions.program`). A bad edit, an orphaning
+delete, or a premature expire **fails the PR build** — never prod.
+
+## Coupling
+
+Low, and deliberately so. The page is a thin container binding store signals to the grid
+organism and mapping the organism's outputs back to store commands
+(`(cellEdited)="store.editCell(…)"`); the organism owns no state (pure input/output).
+Because the editor is schema-driven, a new table adds **no** UI coupling. The only
+editor-side niceties (WP-48) are fast-feedback nudges — `onExpire()` reuses the `CellEdited`
+output to set `geldigTot`; the CI gate stays authoritative.
+
+## See also
+
+- [ADR-0004 — Stamdata as code](architecture/0004-stamdata-as-code.md) — the decision + the org-template exception.
+- [WP-29](../project/backlog/WP-29-stamdata-beheer-editor.md) (editor), [WP-48](../project/backlog/WP-48-stamdata-deletion-protection.md) (deletion protection).
+- `backend/src/BigRegister.Api/Stamdata/StamdataCatalog.cs` — register a table here.
+- `backend/tests/BigRegister.Tests/StamdataValidationTests.cs` — the authoritative gate.
+- `src/app/beheer/ui/stamdata-table-editor/` — the generic, schema-driven grid.
+- [Roles & access](roles-and-access.md) — the `stamdata:edit` capability + admin gating.