fix: lire un document par la requête ancrée, readUnion ayant quitté le polyfill

Le contrat polyfill-surface passe de 1.0.0-dev.6 à 1.0.0-dev.15, épinglé
désormais sur le tag polyfill/v1.0.0-dev.15. readUnion et UnionSubject ont
été retirés en dev.8 : l'application e2e lit un document par un SELECT ancré
regroupé par sujet, en ignorant les registres urn:ng-eventually: que la
requête renvoie désormais.

La déclaration usage_ng-helpers retire ce que l'engagement a absorbé
(exemple de forme, initNg, orm_start_graph corrigé en dev.7, comportement
d'un document illisible, lecture d'un document public par sa seule
référence, pin par tag) et garde les demandes encore ouvertes.

contract_indexing-layer 4.0.1 (patch) : la lecture d'un index ne cite plus
readUnion.

e2e/indexing-app.ts

@@ -15,9 +15,9 @@
  * Nothing here makes an entry of a deposit, and nothing here can ask for one: that is
  * the business of the layer below, and this page is where that shows or does not.
  * Everything else here is the polyfill (`configure`, `ensureIdentity`, `init`, `initNg`,
- * `ng`, `readUnion`, `docs`, `useShapeQuery`) -- including reading the index, which is an
- * ordinary `readUnion` of an ordinary document. If something here is awkward, it is
- * awkward for every consumer, which is the second reason to write it this way.
+ * `ng`, `docs`, `useShapeQuery`) -- including reading the index, which is an ordinary
+ * anchored `docs.sparqlQuery` of an ordinary document. If something here is awkward, it
+ * is awkward for every consumer, which is the second reason to write it this way.
  *
  * This page WIRES THE ORM, because one thing here has no other door: the store enumeration it
  * used to discover a leaked document left the engagement, and what replaces it is a shape
@@ -43,13 +43,11 @@ import {
   init,
   initNg,
   ng,
-  readUnion,
   useShapeQuery,
   type Scope,
   type ShapeObservable,
   type ShapeQuery,
   type ShapeType,
-  type UnionSubject,
 } from "@ng-eventually/polyfill";
 import { ng as realNg, init as realInit } from "@ng-org/web";
 import { RdfOrmSubscription, initNg as ormInitNg } from "@ng-org/orm";
@@ -60,7 +58,13 @@ import { ENTRY_VALUE, INDEX_FIELD, indexing, type Indexing } from "../src/index"
 import { indexingOn } from "../src/indexing";
 import type { NextGraphPort } from "../src/port";
 import { polyfillPort } from "../src/polyfill-adapter";
-import type { BrokenInboxOutcome, IndexRow, IndexingBridge, SelectOutcome } from "./bridge";
+import type {
+  BrokenInboxOutcome,
+  DocumentSubject,
+  IndexRow,
+  IndexingBridge,
+  SelectOutcome,
+} from "./bridge";
 
 // bootstrap: the one polyfill-era call, then the SDK-shaped ones
 //
@@ -82,7 +86,7 @@ configure({
   useShape: () => {
     throw new Error(
       "[e2e] useShape was never wired: this page has no framework adapter to give it, " +
-        "and it reads through readUnion and useShapeQuery instead",
+        "and it reads through docs.sparqlQuery and useShapeQuery instead",
     );
   },
   ormSubscription: RdfOrmSubscription.getOrCreate,
@@ -129,8 +133,8 @@ async function boot(): Promise<void> {
   // through it, and this package's own `initNg` is the one to CALL -- not the SDK's,
   // which was handed to `configure` and is reached from there. The `ng` it is given is
   // this package's too, the one the surface says to call instead of the one `configure`
-  // was passed. Measured: the argument makes no difference either way, because the call
-  // substitutes its own -- see `usage_ng-helpers`'s `## Frictions`.
+  // was passed. The engagement states the argument is discarded either way: the call
+  // substitutes that same published `ng`.
   initNg(ng, session);
   // The session id, and nothing else — this application never names a port, and never
   // awaits anything here: a handle reaches nothing. A reference this application hands
@@ -325,6 +329,40 @@ function rowsOf(result: unknown): Array<Record<string, string>> {
   });
 }
 
+/**
+ * Everything a document holds, grouped by subject -- what an application writes in place
+ * of the `readUnion` the polyfill withdrew in `1.0.0-dev.8`.
+ *
+ * One anchored `SELECT ?s ?p ?o`: the anchor makes the document the query's default graph,
+ * so the body names no graph and reads that document and nothing else. A document this
+ * identity cannot read REJECTS, and the rejection is left to reach the caller -- an
+ * unreadable document is "unknown", never "empty". The polyfill's own registers live
+ * under `urn:ng-eventually:` in the same graph and come back with the rest; they are the
+ * polyfill's, not the document's, so they are dropped here.
+ */
+async function readDocument(doc: string): Promise<DocumentSubject[]> {
+  const session = await sessionReady;
+  const result = await docs.sparqlQuery(
+    session.session_id,
+    "SELECT ?s ?p ?o WHERE { ?s ?p ?o }",
+    undefined,
+    doc,
+  );
+  const bySubject = new Map<string, Record<string, string[]>>();
+  for (const row of rowsOf(result)) {
+    const { s, p, o } = row;
+    if (s === undefined || p === undefined || o === undefined) continue;
+    if (s.startsWith(RESERVED_PREFIX)) continue;
+    const props = bySubject.get(s) ?? {};
+    (props[p] ??= []).push(o);
+    bySubject.set(s, props);
+  }
+  return [...bySubject].map(([subject, props]) => ({ subject, props }));
+}
+
+/** The polyfill's reserved prefix: what it writes into a document under it is its own. */
+const RESERVED_PREFIX = "urn:ng-eventually:";
+
 // the acts
 
 const bridge: IndexingBridge = {
@@ -372,12 +410,12 @@ const bridge: IndexingBridge = {
   /**
    * The index read BY THIS APPLICATION, with nothing the package publishes but
    * `ENTRY_VALUE` — the claim "an index is an ordinary document" performed rather than
-   * repeated. `readUnion([index])` is the same call this page makes on any other
+   * repeated. `readDocument(index)` is the same read this page makes of any other
    * document; the index's own subject is the one that is not an entry, told apart by
    * being the document itself.
    */
   async read(index: string): Promise<IndexRow[]> {
-    const subjects = await readUnion([index]);
+    const subjects = await readDocument(index);
     const rows: IndexRow[] = [];
     for (const subject of subjects) {
       if (subject.subject === index) continue;
@@ -394,8 +432,8 @@ const bridge: IndexingBridge = {
     return rows;
   },
 
-  async readRaw(doc: string): Promise<UnionSubject[]> {
-    return readUnion([doc]);
+  async readRaw(doc: string): Promise<DocumentSubject[]> {
+    return readDocument(doc);
   },
 
   /**