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. »

docs/source-layout-by-fate.md

@@ -1,6 +1,6 @@
 # Source layout by migration fate — analysis and recommendation
 
-**Status: ADOPTED.** The layout this document recommends is the one in the tree — `model/`, `surface/`, `emulated-verifier/`, `shared-wallet/`. Read the rest as the reasoning that produced it, not as a proposal. *(Two things have moved under it since: the package became `packages/sdk` / `@ng-eventually/sdk` on 2026-08-07 — it was `packages/client` / `@ng-eventually/client`, kept here as a search keyword — and `emulated-verifier/public-store.ts` was added on 2026-08-06. Paths below point at where the files are now.)*
+**Status: ADOPTED.** The layout this document recommends is the one in the tree — `model/`, `surface/`, `emulated-verifier/`, `shared-wallet/`. Read the rest as the reasoning that produced it, not as a proposal. *(Two things have moved under it since: the package became `packages/polyfill` / `@ng-eventually/polyfill` on 2026-08-10 — it was `packages/client` / `@ng-eventually/client`, kept here as a search keyword — and `emulated-verifier/public-store.ts` was added on 2026-08-06. Paths below point at where the files are now.)*
 
 Written 2026-08-04, when nothing had yet been moved, from the source of `src/` (25 modules), the two contracts (`docs/api-contract.md`, `docs/internal-contract.md`) used as the export-level inventory, and the read-only `nextgraph-rs` clone (HEAD `213338f6`); the upstream facts this analysis leans on were re-verified at the source and are cited with layer numbers per `README.md` § *The three references* (1 = engine, 2 = wasm binding, 3 = JS ORM).
 
@@ -136,7 +136,7 @@ Full mixed list: `docs.ts`, `surface/read-model.ts`, `inbox.ts`, `subscribe.ts`,
 
 - All 23 non-entry `src/` modules import each other relatively; every moved file churns its importers' paths (mechanical, type-checked).
 - 19 of 19 unit-test files deep-import `../src/*` — 66 static import lines (heaviest: `store-registry` ×16, `polyfill` ×14) **plus 3 dynamic `await import("../src/…")` sites** (`test/isolation-active.test.ts:359`, `test/reach.test.ts:201-202`) that a naive static-import codemod will miss and that fail only at runtime.
-- The e2e harness deep-imports twice (`e2e/sdk-entry.ts:41-42`: `../src/store-registry`, `../src/accounts`); its package-name imports resolve through the exports map and are immune. `e2e/tsconfig.json` includes `"."` only — path-agnostic.
+- The e2e harness deep-imports twice (`e2e/polyfill-entry.ts:41-42`: `../src/store-registry`, `../src/accounts`); its package-name imports resolve through the exports map and are immune. `e2e/tsconfig.json` includes `"."` only — path-agnostic.
 - Decision to make alongside: whether `test/` mirrors the new folders (keeps the module↔spec correspondence at more churn) or stays flat.
 
 **Silent-breakage candidates — the ones tests may not catch:**