feat(scenarios): wire real RAG embedding + sample document set for Glass Box demo mode

[roryford]

Context

The Glass Box P4a flagship scenario (self-managing-research-session) uses FixedCountPreTurnCompressionPolicy with a synthetic memory record and keyword-fallback RAG. The #1531 spec called for wiring a real embedding backend and a sample document library so the demo-mode (live backend) path exercises the full stack — context assembly, retrieval slots, citation provenance, and compression across a real context boundary.

What's missing

• RAGConfiguration in the demo bootstrap is empty → keyword-fallback only, no vector retrieval
• No sample document fixture set in the test/demo library
• Demo mode of the research-session scenario doesn't assert Citation provenance on assistant messages (non-deterministic content was the blocker; structural assertion on at least one citation is achievable)

Deliverables

• Sample document set (3–5 short Markdown files) checked in under Tests/Fixtures/Documents/ or Sources/ManifoldTestSupport/Fixtures/
• RAGConfiguration wired in the Glass Box demo bootstrap with nomic-embed-text (already on disk per memory) as the default embedding backend
• Live-mode structural assertion: at least one Citation attached to an assistant message in the research-session scenario
• Pre-turn compression fires at least once against the real context window (not a fixed count)

Policy

Ships in the research-session scenario PR; docs update in the same PR.