refactor: le paquet s'appelle polyfill, « SDK » désigne celui de NextGraph

Le nom @ng-eventually/sdk entrait en collision avec le SDK de NextGraph, dont
ce paquet est justement un polyfill. Impossible d'écrire « le SDK » sans lever
l'ambiguïté à chaque phrase — et le contrat publié, lu par une application,
était le pire endroit pour laisser traîner ça.

packages/sdk → packages/polyfill, @ng-eventually/sdk → @ng-eventually/polyfill,
contract_sdk-surface → contract_polyfill-surface, e2e/sdk-entry.ts →
e2e/polyfill-entry.ts, docs/sdk-reference.md → docs/polyfill-reference.md.

Les occurrences de « SDK » qui désignent celui de NextGraph restent intactes,
y compris les chemins dans nextgraph-rs (sdk/js/orm, sdk/js/web). Le tri s'est
fait occurrence par occurrence, pas par substitution.

Le contrat énonce désormais son identité en une phrase : « This package is a
polyfill of NextGraph's SDK. »

examples/notebook/app.ts

@@ -1,5 +1,5 @@
 /**
- * Notebook — a minimal application written against `@ng-eventually/sdk`.
+ * Notebook — a minimal application written against `@ng-eventually/polyfill`.
  *
  * It exists for two reasons, and the second is the one that matters:
  *
@@ -7,7 +7,7 @@
  *    writes; there is no test scaffolding, no privileged import, no reaching into the
  *    library's internals. If something is awkward here, it is awkward for everyone.
  *
- * 2. **It is what the applicative e2e suite drives** (`packages/sdk/e2e/notebook.ts`).
+ * 2. **It is what the applicative e2e suite drives** (`packages/polyfill/e2e/notebook.ts`).
  *    The other suite talks to a bag of methods on `window.__sdk`, which proves the
  *    functions run but never that an application could be written with them — and that
  *    gap shipped a real defect: a document's inbox was green in tests and unusable in
@@ -40,7 +40,7 @@ import {
   type Scope,
   // Polyfill-era — ONE call, and it is the whole of what goes away.
   configure,
-} from "@ng-eventually/sdk";
+} from "@ng-eventually/polyfill";
 import { ng as realNg, init as realInit } from "@ng-org/web";
 
 // --- the domain, such as it is ---------------------------------------------