research: XWiki deep dive (impl, extension interfaces, ecosystem); UC-38/39

Deep dive into XWiki past the landscape scan: component/DI architecture,
document + class/object data model, oldcore, Hibernate storage + xwikircs
history, ObservationManager events, rendering pipeline, multi-wiki; the full
extension-interface surface (components, Java/wiki macros, script services,
UIX/UIXP, JAX-RS REST, event listeners, resource handlers); and the
extensions.xwiki.org ecosystem (XAR/JAR/WebJar, 900+ extensions).

Catalog:
- UC-38 make a wiki engine federation-capable via its native extension API
  (composable integration — first engine-side-direction UC; XWiki is proof)
- UC-39 attach a wiki-as-application-platform shard (bodiless typed pages)
- enrich UC-31 (ObservationManager event-driven sync), UC-34 (XObject model),
  UC-36 (xwikircs internal history) with concrete XWiki specifics

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

research/260613-xwiki-deep-dive/README.md

@@ -0,0 +1,35 @@
+# 260613 — XWiki deep dive (implementation, extension interfaces, ecosystem)
+
+Date: 2026-06-13
+
+## What this is
+
+A focused study of **XWiki** — the canonical "wiki-as-application-platform"
+engine flagged in `research/260608-wikiengines-overview/`. Goes past the landscape
+scan into XWiki's **implementation** (component/DI architecture, document data
+model, storage, rendering, events), the **interfaces it exposes to extend the core
+engine** (components, macros, UI extension points, script services, REST), and the
+**extension ecosystem** (extensions.xwiki.org repository, XAR/JAR/WebJar types,
+notable extensions).
+
+Read through one lens: **what does XWiki teach shard-wiki about (a) adapting a
+structured app-platform engine as a shard, and (b) how a host engine can become
+federation-capable through its own extension mechanism** (INTENT *composable
+integration*).
+
+Complements:
+- `research/260608-wikiengines-overview/` — the landscape scan that surfaced XWiki
+- `research/260608-federation-concepts/` — already covered XWiki's ActivityPub extension
+
+## Contents
+
+| Path | Role |
+|------|------|
+| `findings.md` | Architecture, extension-interface catalog, ecosystem, capability profile, INTENT mapping, UC seeds, sources |
+
+## Status
+
+Initial deep dive complete. Two new use cases promoted to
+`spec/UseCaseCatalog.md` (UC-38, UC-39); existing UC-34/UC-36/UC-31 enriched with
+XWiki specifics. Adapter-contract implications feed
+`workplans/SHARD-WP-0002-federation-architecture.md`.