CommonJS model: the depth cutoff fires on known leaves and sends the whole file to the pre-model path #36

Found in the round-1 review of branch fix/commonjs-model-gaps (stacked on PR #29, which fixes #32).

incomplete() returns 'depth cutoff' as soon as depth > 8, before it checks the value's kind. So a fully known leaf (data or a function) nested more than 8 levels deep makes the whole file unmodelled. Prototype chains count toward the same depth.

Repro: exports.a = a; exports.cfg = {l1:{l2:{l3:{l4:{l5:{l6:{l7:{l8:{l9: 1}}}}}}}}}; gives incomplete: depth cutoff, so every member, including a, resolves through the pre-model path.

This is safe, because the fallback is baseline, but it gives up more often than it needs to.

Fix direction: apply the cutoff only when the walk would go into an unexplored identity or union, not at a leaf.