Document Relay @required and export rules

Capture two conventions surfaced while building the top bar: use the
Relay @required directive to make expected-present nullable fields
non-null for consistent typing, and reserve default exports for the
component that lazy() imports as a bundle entry while everything else
uses named exports.

Signed-off-by: Émile Ré <emile@probo.com>

.cursor/rules/react-named-exports-lazy-entry.mdc

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+---
+description: Named exports for components; default export only for lazy() bundle entries
+globs: "**/*.tsx"
+alwaysApply: false
+---
+
+# Named exports; `export default` only for lazy() entries
+
+React components use **named exports**. `export default` is reserved as an
+exception **only** for the component a router/`lazy()` imports as a bundle entry,
+because `lazy(() => import("./X"))` resolves the module's default export.
+
+In the Loader + Page/Layout pattern this means exactly **one** default export per
+chain — the Loader (the `lazy()` target). The Page/Layout it renders is imported
+**directly by the Loader**, not through `lazy()`, so it must be a named export.
+
+```tsx
+// GOOD — Loader is the lazy() entry → default export
+// MainLayoutLoader.tsx
+import { MainLayout, mainLayoutQuery } from "./MainLayout";
+export default function MainLayoutLoader() { /* useQueryLoader → <MainLayout /> */ }
+
+// GOOD — Layout/Page imported directly by the Loader → named export
+// MainLayout.tsx
+export const mainLayoutQuery = graphql`query MainLayoutQuery { ...TopBar_query }`;
+export function MainLayout({ queryRef }: MainLayoutProps) { /* usePreloadedQuery */ }
+
+// routes.tsx — only the Loader is referenced by lazy()
+Component: lazy(() => import("#/pages/MainLayoutLoader")),
+```
+
+```tsx
+// BAD — Page/Layout defaulted even though the Loader (not lazy) imports it
+// MainLayout.tsx
+export default function MainLayout(/* ... */) {}
+// MainLayoutLoader.tsx
+import MainLayout, { mainLayoutQuery } from "./MainLayout";
+```
+
+A page with no loader (no Relay data) that `lazy()` imports directly is itself
+the bundle entry, so it keeps the `export default`. The rule is about *who
+`lazy()` imports*, not about the "Page" vs "Layout" label.
+
+See `contrib/claude/react-components.md` (File and export) and
+`contrib/claude/app-arborescence.md`.

.cursor/rules/relay-required-directive.mdc

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+---
+description: Use Relay @required to make expected-present nullable fields non-null
+globs: "**/*.tsx"
+alwaysApply: false
+---
+
+# Use `@required` for expected-present fields
+
+GraphQL schemas mark many fields nullable defensively, but at a given call site
+you usually **expect** a value. When a field is nullable in the schema but the
+component cannot meaningfully render without it, annotate it with `@required` so
+the **generated type is non-null**. This keeps typing consistent: callers stop
+threading `?.` / `?? ""` / `!` through code that always expects data, and a
+genuinely-missing value becomes a real signal instead of a silently-empty UI.
+
+Choose the action by what should happen when the value is actually absent:
+
+- `@required(action: THROW)` — the value is an invariant for this view (a page's
+  root entity, the `currentTrustCenter` a portal is built around). A null throws
+  on read and propagates to the nearest error boundary. Field becomes non-null.
+- `@required(action: LOG)` — a missing value should degrade gracefully: the null
+  bubbles to the nearest `@required` ancestor (or nulls the fragment data) and
+  Relay logs it. Use when the surrounding UI can render a fallback.
+- `@required(action: NONE)` — bubble nullability without logging; rarely needed.
+
+```graphql
+# GOOD — the view is built around this entity; THROW makes it non-null
+currentTrustCenter @required(action: THROW) {
+  organization {
+    name                    # already String! — no @required needed
+    logo { downloadUrl }    # legitimately optional — leave nullable
+  }
+}
+```
+
+```tsx
+// GOOD — non-null typing falls out of @required; no defensive chaining
+const { organization } = data.currentTrustCenter;
+const logoUrl = organization.logo?.downloadUrl ?? undefined; // logo stays optional
+```
+
+Do NOT use `@required` to silence nullability on fields that are *genuinely*
+optional (avatar, logo, optional description) — those keep their nullable type
+and get a real empty/fallback state. Do NOT use the `THROW` as control flow for
+an expected-empty case (it is an error path, not a branch). Do NOT annotate
+fields the schema already declares non-null (`String!`, `Organization!`).
+
+See `contrib/claude/relay.md` (Fragments → Required fields).