chore(client): retirer la migration legacy + alléger les logs d'accès

- Migration des comptes legacy supprimée (migrateLegacyRecords + garde migratedInto
  + call-sites). Un wallet pré-fix (records store-root, pas de pointeur) provisionne
  simplement un doc-shim frais; contenu legacy ignoré (voulu, données = dev). La
  résolution barrière-autoritative + anti-fork (resolvePointer/ensureRepoOpen/
  canonicalDoc/ensureInFlight/pointerGuard) est inchangée.
- Logs d'accès SDK préfixés [polyfill] + NURI tronqué via shortNuri() (retire
  did:ng:o: et :v:…, garde 8 chars) → moins verbeux.
  Ex: [polyfill] [user1] READ vDlwbZio… (resolvePointer) → 1 triple-rows

Tests: bun test unit 126/0. Docs (nextgraph-current-state/simulation/migration-guide)
mis à jour (migration legacy retirée du modèle décrit).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/migration-guide.md

@@ -47,8 +47,8 @@ The `sharedWalletShim` (account → 3 scope-document NURIs, held in a subscribab
 doc-shim reached via a write-once pointer in the store-root — see
 [`nextgraph-current-state.md`](./nextgraph-current-state.md) § *The pointer → doc-shim
 indirection*) has no target equivalent — the target has no central directory. Remove
-it entirely: `store-registry.ts`, `configureStoreRegistry`, the pointer + doc-shim +
-migration SPARQL, and the `pointerGuard` dep. Cross-wallet reads replace the fan-out;
+it entirely: `store-registry.ts`, `configureStoreRegistry`, the pointer + doc-shim
+resolution, and the `pointerGuard` dep. Cross-wallet reads replace the fan-out;
 per-user wallets replace the shared one.
 
 ### 4. Real inbox → drop the in-lib read emulation