[G3] Gate: certify advanced research workflow

[dkritarth]

Gate purpose

Final planned-depth certification for cross-paper analysis, evidence navigation, writing, and import reliability.

Dependencies

Blocked by #43, #44, #45, and #46, plus every blocker defect discovered during verification.

Exit criteria

• Cross-paper synthesis claims are source-checked on real corpus.
• Citation graph sampled edges have provenance and no known fabricated relation.
• Writing workflow preserves user content and evidence links across failure/restart/export.
• Zotero import is idempotent and preserves existing state.
• Security review covers paper prompt injection, external metadata, file paths, and import data.
• Full suite, typecheck, build, E2E, performance smoke, and diff check pass.
• Independent reviewer posts complete representative workflow evidence.
• All linked blockers are closed.

Closing this gate means planned workflow is verified, not that Vellum has no future work.

Required evidence

Focused PR uses Closes #47. Provide a written design first, failing tests/baseline, focused/full tests, typecheck, build, diff check, exact live Electron procedure, real-paper evidence, screenshots, console state, performance observations, and limitations. Independent reviewer repeats representative workflow.