feat(data): union read model — list via anchorless sparql_query, hang eliminated

Replace the reactive-ORM per-entity fan-out read (which HUNG 75s: orm_start_graph
opened every scope graph and RepoNotFound on any fresh/unsynced doc aborted the
subscription) with the read model:
- readEntities.ts → lib readUnion: resolve the by-need doc set (my own scope docs
  via listMyEntityDocs + public events via the discovery index — NOT all-accounts
  fan-out), then ONE anchorless union sparql_query (GRAPH ?g, VALUES-pinned). Map
  to app types. Re-query on a change signal (no reactive union query).
- countUserParticipations no longer fans out over all accounts (own docs only).
- await loadTestData in the seed step; deleted orphaned useShapeWithDefaults;
  removed the old multistore-stopgap fan-out scenarios; added the read-model-probe.
- Doctrine: rule_document-per-entity read half + _overview rewritten to the union
  model (write half unchanged).

Result: the 75s ORM hang is ELIMINATED (0 hangs; build/tsc/lib-93-tests green;
boundary clean). @data is NOT yet fully green: remaining failures are 90s step
timeouts in the test-harness broker data ops (clearWallet / runUnionProbe / seed)
this run — a harness/broker-op issue, not the read path. To finish separately.

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

.project/concepts/app-architecture/_debt.md

@@ -0,0 +1,7 @@
+# Doc-debt — app-architecture
+
+> Presence of a block = doc to update. Processed → delete the block; no blocks left → delete this file.
+> One block = one "big change": `why` + `files` + `verify` (leaves to review).
+
+## Raw markers (consolidate into blocks, then delete)
+- TOUCHED src/shared/context/FestipodDataContext.tsx @2026-07-05 (session 0b064e8b-1717-421f-a20e-a4318ad217b1)