fix: trois coûts qui revenaient à l'appelant reviennent au paquet

Le contrat faisait porter à l'application trois choses qui sont des artefacts de
notre implémentation, pas de la cible.

Le rechargement de page. Au retour depuis le cache du navigateur, la barrière se
rechargeait pour rejouer init() — et détruisait au passage l'état de
l'application, qui ne pouvait ni s'y opposer ni nettoyer avant. Le paquet
détenait pourtant ce qu'il fallait : la fonction init injectée et le callback de
l'appelant. Il enregistre désormais sa délégation, ranime sa barrière au retour
— champ conservé, bouton réactivé — et redélègue à la confirmation. Rien hors de
la barrière n'est touché. Vérifié dans le bundle amont : en page de tête, init
navigue à chaque appel, sa garde « une seule fois » ne portant que sur la
branche iframe.

L'ordre d'appel silencieux. ensureIdentity() attendu avant init() ne se
résolvait jamais, sans erreur. Le paquet possédant la session, il distingue
maintenant les deux cas sans délai ni heuristique : session pas encore arrivée →
il attend ; init jamais appelé → elle n'arrivera pas, il lève en nommant l'appel
à faire d'abord.

Et la clause qui annonçait la barrière était rangée dans les exigences de
déploiement, alors qu'une application n'y peut rien. Elle passe dans les
garanties, avec ce qui la remplace : la page n'est jamais rechargée.

Il reste deux lignes d'exigences : servir le fichier de portefeuille, et appeler
init avant d'attendre l'identité — ce qui échoue désormais bruyamment.

.project/concepts/app-contract/_debt.md

@@ -0,0 +1,8 @@
+# Doc-debt — app-contract
+
+> Presence of a block = doc to update. Processed → delete the block; no blocks left → delete this file.
+> One block = one "big change": `why` + `files` + `verify` (leaves to review).
+
+## Raw markers (consolidate into blocks, then delete)
+- TOUCHED packages/polyfill/src/surface/lifecycle.ts @2026-08-13 (session f93872b5-293a-4916-a353-181409a96d42)
+- TOUCHED docs/api-contract.md @2026-08-13 (session f93872b5-293a-4916-a353-181409a96d42)

.project/concepts/app-contract/contract_polyfill-surface.md

@@ -16,8 +16,7 @@ This package covers placement (creating and listing an application's documents b
 An application using this package must:
 
 - serve a wallet file (`.ngw`) from its own bundle, and pass its URL and password to `configure` as `sharedWallet: { fileUrl, password }`;
-- call `init(…)` — this package's, not the one it passed to `configure` — and then await `ensureIdentity()`, in a browser context, before rendering its interface. `ensureIdentity()` resolves once a session is open, so awaiting it before `init` has been called never resolves;
-- expect `ensureIdentity()` to mount a full-screen barrier on every top-level load, and the page to reload itself once when a person returns to it: hold no un-persisted state across that call.
+- call `init(…)` — this package's, not the one it passed to `configure` — and then await `ensureIdentity()`, in a browser context, before rendering its interface. `ensureIdentity()` resolves once a session is open, and a session arrives only through `init`: awaited before `init` has been called, it throws and names the call to make first.
 
 ## Surface
 
@@ -109,6 +108,8 @@ Only a document's owner writes to it. Holding its read key never grants a write.
 
 `ensureIdentity()` settles the identity, completes the connection work it starts, and returns the identity. It takes no identifier, and no other call takes one.
 
+`ensureIdentity()` mounts a full-screen barrier on every top-level load, and takes it down itself — past the broker round-trip it never appears. A person who comes back to the page from that round-trip finds the barrier live again, prefilled, and confirming it hands the page over a second time. The application's own page is never reloaded and nothing outside the barrier is touched.
+
 **The session is the package's, not yours.** You never build one, and no call takes one. Call this package's `init` (not the one you passed to `configure`): it captures the session the SDK delivers to `init`'s callback and keeps it, then calls your callback with that same event untouched — so an application that wants the `session_id` for the `docs` primitives reads it there, and one that does not may pass no callback at all. Identity normalisation is the package's too: `@Alice`, `alice ` and `ALICE` are one person.
 
 `createEntityDoc` throws if the document cannot be recorded in its store.

.project/concepts/sign-in/_debt.md

@@ -0,0 +1,10 @@
+# Doc-debt — sign-in
+
+> Presence of a block = doc to update. Processed → delete the block; no blocks left → delete this file.
+> One block = one "big change": `why` + `files` + `verify` (leaves to review).
+
+## Raw markers (consolidate into blocks, then delete)
+- TOUCHED packages/polyfill/src/shared-wallet/bootstrap.ts @2026-08-13 (session f93872b5-293a-4916-a353-181409a96d42)
+- TOUCHED packages/polyfill/src/shared-wallet/access-gate.ts @2026-08-13 (session f93872b5-293a-4916-a353-181409a96d42)
+- TOUCHED packages/polyfill/src/surface/lifecycle.ts @2026-08-13 (session f93872b5-293a-4916-a353-181409a96d42)
+- TOUCHED packages/polyfill/src/shared-wallet/session.ts @2026-08-13 (session f93872b5-293a-4916-a353-181409a96d42)