SearXNG joins as a peer provider — social packs default to the free metasearch container; Tavily stays for content-RAG; provider_override seam wired through the stack

The reframe that took shape in late May lands as code: search-provider choice is now an architectural axis of the stack, not a Tavily-vs-anything-else swap. SearXNG arrives as a self-hosted container peer to Tavily, with its own connector under a new connector registry; the common-seven social packs flip their default to SearXNG (no key needed); Tavily stays wired in as the peer for content-RAG packs that need it; and a `provider_override` parameter threads from the response-reviewer's per-record buttons through workspace and social-search so the user can fire any pack through either provider on any row without touching the pack definition. The pre-flight surface for per-row iteration is now in place — the iteration loop itself still pending.