Docs: new-entity persistence stack, modal size tiers, spec anchor stability, test harness notes

docs/architecture/03-modal-system.md

@@ -610,12 +610,16 @@ share the open/close/snapshot/URL logic rather than reimplementing it.
 
 1. Add the name to the `ModalName` union.
 2. Add one `MODAL_REGISTRY` entry (title, component; `getState`/`hasError`/
-   `getError` if it edits; `isUrlNavigable` + `init(arg)` if navigable).
+   `getError` if it edits; `isUrlNavigable` + `init(arg)` if navigable;
+   `dismissOnBackdrop: false` if it holds in-progress work).
 3. Write the body component; it reads/writes its feature store (e.g.
    `useDatasetStore`, `useChartBuilderStore`) via a narrow selector.
-4. If navigable, add its hash form to the URL sync and restore logic.
-5. If it has a keyboard shortcut or workflow trigger, wire that to
+4. If it isn't a small form, add its name to the **size-tier mapping in
+   `ModalShell`** (`isLarge`/`isXLarge`) — the one shell edit a new modal can
+   need; without it the modal renders at the small-form size.
+5. If navigable, add its hash form to the URL sync and restore logic.
+6. If it has a keyboard shortcut or workflow trigger, wire that to
    `openModal(name, arg)` — never to `activeModal` directly.
 
-No edits to the shell render, the close logic, or the change-detection code are
-needed: those are generic and driven entirely by the registry.
+The shell render, close logic, and change detection need no edits: those are
+generic and driven entirely by the registry.