DR-040 — Markdown rendering pivots to Streamdown; new surfaces first

Retired/tombstoned S504. Re-homed (S504 register sweep): a UI convention — the register’s own routing sends how-to conventions to the owning CLAUDE.md. Substance home: canonical:components/CLAUDE.md (markdown rendering). Original ADR text retained below; this id stays on disk so citations resolve and the number is never re-issued. Any UI work that renders markdown uses Streamdown — new surfaces first, per the title. Grandfathered exception: the vanilla-TS MCP apps (mcp-apps/reorient-me) use marked+DOMPurify (ratified by retired DR-083).