research: Foswiki deep dive (store abstraction, ext API); UC-42/43

Deep dive into Foswiki focused on its deltas from TWiki (not the shared
lineage): the pluggable Foswiki::Store backend (RcsWrap/RcsLite/PlainFile)
behind a versioned interface via Foswiki::Meta, the OO/MVC core rewrite,
Foswiki::Func + registerTagHandler, DataForms + MetaDataPlugin multi-record,
and WysiwygPlugin TML<->HTML round-trip. The store abstraction is logged as
prior art for shard-wiki's own adapter contract (SHARD-WP-0002).

Catalog (now 43 UCs):
- UC-42 read/write a non-Markdown shard via lossless syntax translation
  (Markdown-first for prose; Foswiki WysiwygPlugin is proof)
- UC-43 tolerate a shard's storage-backend swap (RCS<->PlainFile) under a
  stable identity
- enrich UC-39 (multi-record metadata) and UC-40 (PlainFile direct-attach)

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

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

@@ -0,0 +1,36 @@
+# 260613 — Foswiki deep dive (store abstraction, extension API, ecosystem)
+
+Date: 2026-06-13
+
+## What this is
+
+A focused study of **Foswiki** — the community fork of TWiki (2008, from TWiki
+4.2.x) — concentrating on **what Foswiki changed**, not the lineage it shares with
+TWiki. The distinctive material: a **pluggable `Foswiki::Store` backend**
+(RcsWrap / RcsLite / **PlainFileStoreContrib**) behind a versioned interface, the
+`Foswiki::Meta` topic-object indirection, the OO/MVC core rewrite, the cleaner
+`Foswiki::Func` + `registerTagHandler` extension API, **DataForms** (+ MetaDataPlugin,
+FlexFormPlugin), and the **WysiwygPlugin** TML↔HTML round-trip.
+
+Read through shard-wiki's lens: Foswiki is the engine that **already separates its
+store backend from its core via a versioned interface** — i.e. it is concrete prior
+art for shard-wiki's own *shard adapter contract* and *capability-aware adapters*.
+
+Pairs with — and deliberately does not repeat — the shared TWiki material:
+- `research/260613-twiki-deep-dive/` — flat-file/RCS store, Webs/Topics, plugin
+  handler callbacks, per-topic ACL (all inherited; read there for the shared parts)
+- `research/260613-xwiki-deep-dive/` — the Java/DB app-platform sibling
+- `research/260608-wikiengines-overview/` — landscape scan
+
+## Contents
+
+| Path | Role |
+|------|------|
+| `findings.md` | Distinctives vs TWiki, store abstraction, extension API, DataForms, capability profile, INTENT mapping, UC seeds, sources |
+
+## Status
+
+Initial deep dive complete. Two new use cases promoted to
+`spec/UseCaseCatalog.md` (UC-42, UC-43); UC-34/39/40 enriched. The
+`Foswiki::Store`/`Foswiki::Meta` abstraction is logged as adapter-contract prior art
+for `workplans/SHARD-WP-0002-federation-architecture.md`.