L2 grouping: the planner and the plan, plan_get, plan_clear verbs #185

Lane L2. Pure planner from pins and costs; segments of consecutive members; reason code per member left alone; chain place chain:.; setting grouping (default off); determinism test and one test per reason.

Design: https://github.com/FreeMixer/plugin-hostd/blob/main/docs/design/notes/2026-10-27-grouping-planner.md (PR #183). Part of #182, FreeMixer/plugin-qualify#157, FreeMixer/openmixer#2263. Operator priority 2026-10-27. One repo, one small change, with its test and the sabotage that reds it.