asap_planner JSON drops metric-type warnings before Stage 3 #1202

`asap_planner --format json` puts warnings under `stage3_plan_selection.warnings` only. With `--run-until` before `stage3`, or when Pass 1 rejects a query (Stage 3 is `null`), the metric-type warnings the text output prints are not in the document.

Fix: a top-level `warnings` array in the document (and the viewer reads it there).

Found in review of #1190 (#1190 (comment)).

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