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/concepts/app-contract/_overview.md

@@ -1,19 +1,19 @@
 ---
 type: overview
-summary: What an application may rely on from @ng-eventually/sdk, and what it will have to delete
+summary: What an application may rely on from @ng-eventually/polyfill, and what it will have to delete
 triggers:
   keywords: [polyfill, sdk, surface, contract, publish, published, entry, export, migration, unlearn, consumer, app-facing]
   paths:
-    - "packages/sdk/src/index.ts"
-    - "packages/sdk/src/surface/**"
-    - "packages/sdk/README.md"
+    - "packages/polyfill/src/index.ts"
+    - "packages/polyfill/src/surface/**"
+    - "packages/polyfill/README.md"
     - "examples/notebook/**"
     - "docs/api-contract.md"
 vocabulary:
   - term: reference
     gloss: a NURI that names a document and grants nothing — what an application circulates
     not: [link, lien, share-link]
-    see: contract_sdk-surface
+    see: contract_polyfill-surface
   - term: ReadCap
     gloss: upstream's word for what opens a document — a reference carrying its secret
     not: [token, credential, permission]
@@ -29,10 +29,10 @@ This library exists so an application can be **written today against the NextGra
 
 The distinguishing question, asked at every choice — published signature as much as internal mechanism: **does the target do this?** If it does it differently, we do it their way, whatever that costs. *Would a caller have to unlearn it?* comes second, and measures how bad a divergence is — never whether one is allowed. See `rule_no-divergence-from-nextgraph`.
 
-This repo is the **provider** of `contract_sdk-surface`; consuming applications live in other repos and pull it. The per-symbol ruling, with an epistemic label on every target-side claim, stays here in `docs/api-contract.md` — that is maintainer material, not the engagement.
+This repo is the **provider** of `contract_polyfill-surface`; consuming applications live in other repos and pull it. The per-symbol ruling, with an epistemic label on every target-side claim, stays here in `docs/api-contract.md` — that is maintainer material, not the engagement.
 
 ## Read first
 
-- `contract_sdk-surface` — the engagement itself, written from the caller's point of view.
+- `contract_polyfill-surface` — the engagement itself, written from the caller's point of view.
 - `rule_no-divergence-from-nextgraph` — the test that decides what may be built at all.
 - `knowledge_what-an-app-deletes-at-migration` — the two fates a published symbol can have.