Adds Fusion as a third combo strategy alongside fallback/round-robin. A
fusion combo fans the prompt out to all member models in parallel, then a
configurable judge model synthesizes one final answer from the panel.
- handleFusionChat in open-sse/services/combo.js: quorum-grace collection
caps the straggler penalty, anonymized sources prevent judge brand-bias,
degrades to a direct answer on single survivor and 503 on total failure.
- chat.js dispatches strategy==="fusion" at both combo entry points.
- Combos dashboard: per-combo strategy Select replaces the round-robin
toggle, fusion reveals a judge picker, plus a strategy/capacity explainer.
- tests/unit/combo-fusion.test.js covers fan-out, judge routing/default,
quorum-grace straggler drop, single-survivor and total-failure degradation.
Co-authored-by: Cursor <cursoragent@cursor.com>
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