docs(concepts): NextGraph multi-user design — ng-eventually polyfill, discovery, apps/services

Captures the design worked out this session:
- decision: ng-eventually generic polyfill library (external repo) encapsulates all
  multi-user compensation; @ng-eventually/client is SDK-identical, app depends only on it.
- decision: discovery via a single global index fed through its inbox (owned doc,
  materialized) — no Group store; index owner = open question (singleton app, deferred).
- knowledge: NextGraph apps/services are mono-user with no global data (corrects the
  earlier 'index service with its own wallet' model).
- reconciled shared-wallet-shim brief (per-entity docs, login flow, polyfill terminology),
  authorization-matrix (no Group store), data-layer stack (ng-eventually indirection).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.project/concepts/data-layer/knowledge_nextgraph-stack.md

@@ -14,6 +14,8 @@ summary: Paquets @ng-org/* (web, orm, shex-orm, alien-deepsignals), shapes SHEX
 
 Installés depuis npm (`@ng-org/*`, versions alpha). Pour développer contre un build local non publié de `nextgraph-rs`, `scripts/build-ng-packages.sh` pack le monorepo en tarballs et repointe `package.json` (cf. `nextgraph-platform` — le pattern d'origine du projet, réactivable pour un fork).
 
+> **Indirection via `ng-eventually` (depuis 2026-06-22).** Le data-plane ne consomme plus le SDK directement : `useShape` est importé de **`@ng-eventually/client`** (wrapper SDK-identique), et `ngSession` injecte le vrai SDK dans la lib via `configure()` (`@ng-eventually/client/polyfill`). Aujourd'hui la lib **forwarde tout** (passthrough) — comportement identique, validé `@data`. Détails et raison d'être : [[decision_2026-06-17_eventually-library]]. Les imports **de types** (`ShapeType`, `DeepSignalSet`…) restent sur `@ng-org/*`.
+
 ## Shapes SHEX
 
 `src/shared/shapes/shex/festipodShapes.shex` définit :