Improve research RAG retrieval fusion

[jamilahmadzai]

Bounty: ISAAC-497
Algora bounty: https://algora.io/isaac/bounties/clq18zr98000ejs0gt0nv7gwu

Summary:

• Add a research RAG helper for scientific chunk metadata, deterministic query variants, rank fusion, bounded evidence context, and source manifests.
• Update PDF ingestion to store stable citation metadata and use scientific section-aware chunk separators.
• Update document retrieval and RAG chat to use same-origin retrieval, fused evidence, exact citation keys, and the configured temperature.

Validation:

• npx vitest run
• npx tsc --noEmit
• npm run lint
• npm run build

Payout:
Algora bounty-platform payout to GitHub user @jamilahmadzai.