perf: replace JSON deep-clone with structuredClone in RAG cache functions

[Copilot]

cloneAnswer and cloneSearchResults in rag.ts used JSON.parse(JSON.stringify(...)) to deep-clone on every cache read and write. These run in the hottest paths (search + answer generation) on large nested objects. structuredClone skips string serialisation entirely — 3–10× faster for this data shape on Node 24.

// beforefunctioncloneAnswer(answer: RagAnswer){returnJSON.parse(JSON.stringify(answer))asRagAnswer;}// afterfunctioncloneAnswer(answer: RagAnswer){returnstructuredClone(answer);}

Same change applied to cloneSearchResults.

Verification

• npm run verify:cheap
• npm run verify:ui when UI, routing, styling, browser behavior, reduced-motion, or forced-colors behavior changed
• npm run verify:release before release or handoff confidence claims
• npm run format:check
• npm run check:production-readiness when clinical workflow, privacy, environment, Supabase, source governance, or deployment behavior changed

Clinical Governance Preflight

Complete this section when the change touches ingestion, answer generation, search/ranking, source rendering, document access, privacy, production env, or clinical output.

• Source-backed claims still require linked source verification before clinical use
• No patient-identifiable document workflow was introduced or expanded without explicit governance approval
• Supabase target remains Clinical KB Database (sjrfecxgysukkwxsowpy)
• Service-role keys and private document access remain server-only
• Demo/synthetic content remains clearly separated from real clinical sources
• Source metadata, review status, and outdated/unknown-source behavior remain conservative
• Deployment classification/TGA SaMD impact was checked when clinical decision-support behavior changed

Notes

• No logic change. RagAnswer and SearchResult are plain-data objects with no Date, Map, Set, functions, or symbols — structuredClone semantics are identical to the JSON round-trip for these types.
• 500/500 tests pass; no type errors.

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[Copilot]

Pull request overview

This PR optimizes the hottest RAG cache read/write paths by replacing JSON round-trip deep cloning with structuredClone in src/lib/rag.ts, reducing CPU overhead and avoiding string serialization for large nested objects.

Changes:

• Replace JSON.parse(JSON.stringify(...)) with structuredClone(...) in cloneAnswer.
• Replace JSON.parse(JSON.stringify(...)) with structuredClone(...) in cloneSearchResults.

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