Grow the territory tree in linear time for thousands of sibling folders #82

Problem

Growing the territory tree takes longer than linear time when a directory has thousands of sibling folders. On generated repositories of N sibling modules, each with three source files and a placed test, N = 4000 takes about 5 s and N = 8000 about 28 s.

The bucket chain is the cause. When a directory has more than 8 children, 7 open and the rest wait in a "smaller folders" bucket; every split of that bucket opens 7 more and leaves a smaller bucket. N siblings therefore take about N/7 successive splits, and each split re-plans the remaining bucket, so the total work grows quadratically (rankSplits in packages/engine/src/territories/grow-tree.ts, openKids in open-kids.ts).

This predates the territory fixes; it is the remaining super-linear path in territory building.

Proposal

Cut a bucket into groups of 7 in one pass instead of chaining, or cap the chain and let the last bucket stay whole. Sort and weigh the siblings once per directory, and keep the per-folder heat and change counts so a re-plan does not recompute them.

Done when

• A generated repository of 8000 sibling modules builds its territories in under 5 s.
• The existing territory tests still pass.
• A scaling test guards the bound.