fix(inbox): une inbox appartient à un document, jamais à plusieurs

Retour sur l'adresse par défaut livrée en 8a382f2, qui faisait pointer tout
document vers l'inbox de son propriétaire. C'était acheter le coût au prix de
la forme — le mauvais arbitrage pour cette bibliothèque.

Vérifié en amont : le verifier route un message entrant par
`inboxes: PubKey → RepoId` (`engine/verifier/src/verifier.rs:1677,1928`) et le
déchiffre avec la moitié privée de CE repo. Et `InboxMsgBody`
(`engine/net/src/types.rs:4265`) ne porte aucun document cible — il n'en a pas
besoin : l'adresse EST l'identification. Une inbox appartient donc à exactement
un repo, et faire tenir plusieurs documents derrière une inbox émule une
relation que le modèle ne peut pas exprimer.

Conséquences :

- `createEntityDoc` ne publie plus rien. Un document neuf n'a pas d'inbox et
  `documentInboxAddress` rend `undefined`.
- Une inbox s'ouvre par `openDocumentInbox(doc)`, sur décision du propriétaire.
  C'est aussi ce qui règle le coût sans toucher à la forme : seuls les
  documents destinés à RECEVOIR en paient une — l'app le sait, la bibliothèque
  non.
- `inbox.postToDocument(doc, { payload })` : l'app nomme le DOCUMENT, jamais une
  inbox. Lève quand le document n'en a pas, au lieu de rendre la main
  silencieusement — un dépôt qui disparaît sans erreur est exactement le bug que
  ce chemin traînait.
- Pas de champ « document cible » sur un dépôt. Ce serait une invention que les
  apps devraient désapprendre à la migration.

README, principe de conception : les deux moitiés sont contraignantes, et c'est
la seconde qu'on brade. La surface doit être au plus près du futur SDK, mais
l'IMPLÉMENTATION aussi doit être au plus près de ce que NextGraph prévoit, sans
exception. Ce qui est connu vaut spécification. La pression à dévier ne se
présente jamais comme une déviation : elle arrive comme un coût, une latence,
une gêne d'ergonomie — bien réels. Deux cas déjà rencontrés sont consignés, avec
le signal commun : un choix qui ferait apprendre au consommateur quelque chose
qu'il devra DÉSAPPRENDRE.

157 tests unitaires, e2e 40/40 contre le broker en ligne.

docs/briefs/2026-08-03-document-inbox-addressing.md

@@ -6,9 +6,13 @@
 >
 > `storeRegistry.documentInboxAddress(doc)` answers *"where do I deposit for this document"* for **any holder**, and `inbox.post` into it. The address is published **at creation**, in a compartment the library owns — so it never enters a consumer shape.
 >
-> **On the cost.** The measured regression (9m37 → 21m30) came from creating a second **DOCUMENT** per document. The published address points at the owner's **own inbox**, which already exists and is amortized over every document that owner creates: creation grows by triples, not by a document. The deposit carries the document it concerns, so the owner still materializes per document. Shape 1 (derivation) was **not available**: our inbox is a document, and a derived NURI would name a repo `doc_create` never created — upstream can derive because an inbox there is a keypair on the repo, not a document.
+> **On the cost — and the wrong answer that was tried first.** The measured regression (9m37 → 21m30) came from creating a second **DOCUMENT** per document. The first fix pointed every document's published address at the owner's **own** inbox: no second document, cost amortized. **It was reverted the same day**, because it emulates a relation upstream cannot express — the verifier routes an incoming message by `inboxes: PubKey → RepoId` and unseals it with THAT repo's private half (`engine/verifier/src/verifier.rs:1677,1928`), and `InboxMsgBody` carries no target document (`engine/net/src/types.rs:4265`) because the address already identifies it. Many documents behind one inbox would have forced consumers to tag deposits with their document — a habit to unlearn at migration, which is precisely what this library exists to prevent.
 >
-> **Why at creation and not at first open.** Publishing the day the owner opens a dedicated inbox leaves a window where a third party reads the document, finds no address, and cannot reach the owner at all — which is exactly the consumer's central act (signing up to someone else's document, before that owner ever touched an inbox). `openDocumentInbox(doc)` remains, for an owner who wants one document's deposits kept apart; it **replaces** the published address rather than adding to it.
+> **The cost, actually answered:** only documents meant to RECEIVE open an inbox, and their owner is who knows. `createEntityDoc` publishes nothing; an app calls `openDocumentInbox(doc)` for the documents that need one (in the consumer's case: events, not every entity). Cost becomes proportional to the need, with the shape intact.
+>
+> Shape 1 of this brief (derivation) was **not available**: our inbox is a document, and a derived NURI would name a repo `doc_create` never created — upstream can derive because an inbox there is a keypair on the repo, not a document.
+>
+> **`inbox.postToDocument(doc, { payload })`** is the one call an app makes: it names the DOCUMENT, never an inbox, and **throws** when the document has no inbox rather than returning quietly — a deposit that vanishes without an error is the bug this whole path exists to close. There is deliberately **no target-document field on a deposit**, for the reason above.
 >
 > **Where the address lives.** On the document's emulated **Header branch** (`urn:ng-eventually:shim:headerBranch`), beside the content rather than in it — the same subject-as-compartment shape already used for the Store and User branches. `read-model` now drops every subject under the reserved `urn:ng-eventually:` namespace (`src/machinery.ts`), so the address cannot surface as one of the entity's properties. That filter is by SUBJECT, so it covers every emulated compartment present and future.
 >

docs/simulation.md

@@ -217,16 +217,23 @@ store-id:
   resolution answered *"which inbox do I know for this document"*, so a depositor got
   one of their own and their deposit vanished silently
   ([`briefs/2026-08-03-document-inbox-addressing.md`](./briefs/2026-08-03-document-inbox-addressing.md)).
-  Now every document carries its address **from creation**, on its emulated **Header
-  branch** — a reserved subject inside the document, so any holder of the document
-  reads it, and `read-model` filters the whole `urn:ng-eventually:` namespace out of
-  consumer data (`src/machinery.ts`). It points at the owner's own inbox by default —
-  one inbox per user, amortized, NOT one document per document — and
-  `openDocumentInbox` replaces it when an owner wants a document's deposits kept apart.
-  This mirrors upstream's split: a depositor seals with the inbox PUBLIC key and needs
-  nothing else, only the owner holds the private half. At migration the address becomes
-  the repo's native inbox pubkey and the resolution moves; the consumer-facing act
-  (resolve, then `inbox.post`) is unchanged.
+  A document that has an inbox carries its address on its emulated **Header branch** —
+  a reserved subject inside the document, so any holder of the document reads it, and
+  `read-model` filters the whole `urn:ng-eventually:` namespace out of consumer data
+  (`src/machinery.ts`). This mirrors upstream's split: a depositor seals with the inbox
+  PUBLIC key and needs nothing else, only the owner holds the private half.
+  **One inbox belongs to one document** — never several documents behind one inbox, a
+  relation upstream cannot express (the verifier routes by `inboxes: PubKey → RepoId`
+  and unseals with that repo's key, `engine/verifier/src/verifier.rs:1677,1928`), which
+  is also why a deposit carries no target document: the address identifies it. A fresh
+  document therefore has NO inbox and `documentInboxAddress` returns `undefined` — its
+  owner opens one when the document is meant to receive, which is what keeps the cost
+  proportional. At migration the address becomes the repo's native inbox pubkey and the
+  resolution moves; the consumer-facing act is unchanged.
+- **`inbox.postToDocument(doc, { payload })`** — the one call an app makes to reach a
+  document's owner: it names the DOCUMENT, never an inbox. **Throws** when the document
+  has no inbox, rather than returning quietly: a deposit that vanishes without an error
+  is the exact bug this path shipped with.
 
 Both resolve the native store ids from the injected session
 (`RegistrySession.protectedStoreId` / `publicStoreId`, alongside the existing