Follow-ups from the EF-344 final whole-branch review (native LINQ query rewrite sub-project SP6, epic EF-322). EF-344 shipped native GroupBy(key).Select(aggregate) → $group; these are non-blocking items deliberately deferred out of that branch (the branch was merge-ready without them).

Coverage gaps (correct results today via driver-LINQ fallback; candidates to make native):

• Non-member / computed grouping keys fall back. NativeGroupByBinder.TryBindGroupKey accepts only a plain member access or an anonymous/new of member accesses. Keys like EF.Property<T>(o, "X") (a MethodCallExpression) and computed keys (g => g.OrderDate.Year) fall back (correct results). Making EF.Property keys and/or computed keys native is a follow-on.
• User-supplied GroupBy(key, resultSelector) overload declines. The QMTEV TranslateGroupBy marks non-native when resultSelector != null, so the fused overload falls back. Supporting it natively is a follow-on.
• Decline discriminator is Join-family only. MarkGroupByFallbackUnsafe() fires for a group-first GroupBy combined with Join/GroupJoin/LeftJoin (the one shape where driver-LINQ returned silently wrong data). Other correlated operators over a group-first source (SelectMany/set-ops/Concat) are NOT marked fallback-unsafe. No such shape is currently known to return wrong data via the fallback, but if one is found it needs the same decline treatment — residual "too-narrow" risk worth a targeted audit.

Test / code nits:

• Add a functional MongoQueryMode.NativeOnly test that Min/Max/Average group accumulators go native (currently covered by binder unit tests + the MONGODB_EF_NATIVE_ONLY spec sweep, but no dedicated functional NativeOnly case alongside the existing Count/Sum ones).
• NativeGroupByBinder.TryBindGroupProjection has an unused groupingParameterShaper parameter (retained for signature stability during EF-344; it's a static internal helper, so it can just be dropped).
• The binder recomputes composite-key-ness inline (isComposite) instead of reusing MongoGrouping.IsCompositeKey.

Context: design docs/superpowers/specs/2026-07-08-native-groupby-design.md; the as-built behavior is documented in src/MongoDB.EntityFrameworkCore/Query/AGENTS.md (EF-SP6 GroupBy note).