docs: place VerisimCore in the stack diagram (maintainer-confirmed)

Confirmed correct by the maintainer 2026-06-19: the architecture ladder is
KRL (surface) -> TangleIR (canonical representation) -> VerisimCore (categorical
abstraction) -> Skein / QuandleDB (storage and equivalence). Resolves the
previously-parked VerisimCore placement question.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017TXizM5c1Yd9HWf7Y15YH2

Author: claude

README.adoc

@@ -86,13 +86,13 @@ Tangle is part of a layered system:
 
 ---
 
-KRL (surface language — resolution DSL, not merely a query language)
+KRL (surface language)
 ↓
-TangleIR (canonical interchange representation)
+TangleIR (canonical representation)
 ↓
-Tangle core (this repo — proven type-safe small-step semantics)
+VerisimCore (categorical abstraction)
 ↓
-QuandleDB + Skein.jl (persistence + invariant/equivalence; computational engine)
+Skein / QuandleDB (storage and equivalence)
 -------------------------------------------
 
 * *TangleIR* is the central representation shared across layers