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. »

packages/polyfill/src/emulated-verifier/machinery.ts

@@ -0,0 +1,42 @@
+/**
+ * The namespace this library reserves for its OWN triples, and the one predicate a
+ * read path needs about it: *is this subject machinery, or is it the consumer's data?*
+ *
+ * ── Why this exists ────────────────────────────────────────────────────────
+ * The polyfill has no branches, so it emulates each of a repo's compartments with a
+ * distinct SUBJECT inside a document (`shim:index` for the store's Main branch,
+ * `shim:storeBranch`, `shim:userBranch`, `shim:headerBranch` — see `store-registry.ts`).
+ * That was invisible as long as those subjects only ever appeared in documents the
+ * consumer never reads through the data path — store documents and the doc-shim.
+ *
+ * The Header branch broke that: it lives in an ENTITY document, the one the consumer
+ * reads with `SELECT ?s ?p ?o`. Without a filter, the address of a document's inbox
+ * would surface as one of that entity's properties — machinery leaking into domain
+ * data. Filtering by SUBJECT rather than by predicate is what makes this hold for
+ * every compartment, present and future: a new emulated branch needs no new filter.
+ *
+ * Upstream this problem does not exist, because there the separation is real — a
+ * branch is a different CRDT with its own topic, not a subject in the same graph. This
+ * module is the seam where our emulation pays for that.
+ */
+
+/**
+ * The URN namespace every triple this library writes for itself lives under —
+ * `urn:ng-eventually:shim:…` (store-registry's compartments) and
+ * `urn:ng-eventually:inbox:…` (inbox deposits).
+ *
+ * A consumer that writes its own data under this prefix would have it filtered out of
+ * its reads. That is a deliberate reservation, not a hazard to guard against: the
+ * namespace names this library.
+ */
+export const MACHINERY_NS = "urn:ng-eventually:";
+
+/**
+ * Is `subject` one of this library's own, rather than consumer data?
+ *
+ * Tolerant of `undefined` so a read path can hand it a possibly-absent binding
+ * without a preliminary check — an absent subject is not machinery.
+ */
+export function isMachinerySubject(subject: string | undefined): boolean {
+  return subject !== undefined && subject.startsWith(MACHINERY_NS);
+}