jev_decide: surface contradicted requirements, add opt-in escalation

[Rex-Gao]

Fixes #43. This includes the structural half of #44 (recommendation.contradicted_requirements, same shape: zero-based requirement indexes from contradictsRecommendation) and adds the opt-in escalation half that issue left as a maintainer decision — happy to defer to #44 for the structural piece if that lands first.

What

• recommendation.contradicted_requirements: number[] now ships on every jev_decide result (empty when nothing contradicted, [] on invalid_response), so the disagreement is structural, not a warning string.
• New optional input escalate_on_contradiction (default false, current behavior unchanged): when set, a recommendation whose own requirement checks came back contradicted returns selected: null with status: "escalate" — the jev_verify vocabulary — instead of the candidate id. probabilities and contradicted_requirements stay inspectable.
• No re-selection, per option 3's fail-closed argument in the issue: a withdrawn recommendation is never silently replaced by the runner-up; the model's genuine pick stays visible next to the escalation.

Testing

npm run typecheck, npm run build, npm test: 245 tests. New coverage: default keeps selected with the structural array and the warning; escalate_on_contradiction withdraws a contradicted pick without promoting the runner-up; the flag keeps a non-contradicted recommendation untouched (a contradiction on a different candidate never escalates).

Docs

README jev_decide section: result example gains the field, and a bullet documents the flag and the no-re-selection rule.

[jkudish]

Thank you — merged, with one docs amendment: escalation happens in addition to the warning, and the indexes describe the pick before withdrawal. Shipping in the next release.