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. »

.project/contracts.yaml

@@ -1,11 +1,11 @@
 # Inter-repo contracts. `publish:` is this project's engagement toward its consumers —
 # listing a leaf here IS the act of publishing it; an unlisted `contract_` leaf is a draft.
 #
-# This project is a PROVIDER only for now: the applications that consume the SDK live in
-# their own repos and pull `sdk-surface` from here. Nothing is consumed in the other
+# This project is a PROVIDER only for now: the applications that consume the polyfill live in
+# their own repos and pull `polyfill-surface` from here. Nothing is consumed in the other
 # direction — the NextGraph clone this library aligns on is a read-only reference we
 # verify against, not a contract anyone stamps.
 
 publish:
   # paths are relative to `.project/`
-  sdk-surface: concepts/app-contract/contract_sdk-surface.md
+  polyfill-surface: concepts/app-contract/contract_polyfill-surface.md