docs: multi-persistence architecture research (options + recommendation)

Fable research per the multi-persistence concern: metamodel half is already
decided by ADR-0007 (store-paradigm subtypes + @ROLE); the gaps are single-store
rails and a paradigm-blind persistability check in migrate-ts. Recommendation is
a paradigm-scoped hasWritableSource(paradigm) predicate + a new ADR now, first
non-RDB paradigm (search) post-1.0. Research artifact for review — not a decision.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HLoJkFSyoticveo5ehMUAr

Author: dmealing