A new docs/core-data-structures/ folder: a self-contained core.md defining what
counts as a "core" data structure (the agent-loop spine) and covering the spine
vocabulary, plus per-seam sub-pages (llm-streaming, session, persistence, tools,
bash). Type definitions are pasted verbatim via `ts type-equiv` blocks and
drift-checked by verify-type-equiv. Cross-linked from architecture.md; the
`ts type-equiv` mechanics are documented in development.md.
