COUNT_SCAN is the only stage whose legacy explain output emits its execution counters (keysExamined, peakTrackedMemBytes) before its structural fields (keyPattern, indexName, indexBounds, ...); every other stage emits structure first. The V3 explain format is unaffected (its serializer orders all stages uniformly).

The quirk forces a permanent special case in the legacy per-stage emission composition in plan_explainer_impl.cpp (see the TODO referencing this ticket), where the structure and counter emitters are composed in per-stage order to keep legacy output byte-identical.

Proposal: normalize legacy COUNT_SCAN explain field order to structure-first, like every other stage. This is an order-only change - same fields, same values - so name-based consumers are unaffected; a core-suite run should flag any order-sensitive tests. Once landed, the composition special case can be deleted.

Out of scope of SERVER-130529 by its own constraints (legacy output byte-stable).