Decouple V2 dependencies from query path

[em3s]

Background

The query path (V3QueryService, ActionbaseQueryExecutor, LabelProvider, CompatKt) directly depends on 36 V2 types. The mutation path was already decoupled in #201 using the MutationEngine interface + V2BackedEngine pattern. Apply the same pattern to queries.

Task

• Decouple like Extract MutationEngine and TableBinding abstractions #201

Done When

• boundary-check: 0 V2 edges for ActionbaseQueryExecutor, LabelProvider (deleted), CompatKt (deleted)
• All V2 types confined to V2Backed* classes
• All existing tests pass

Notes

• V2 types in ActionbaseQuery itself (DataType, StatKey, WherePredicate) are out of scope — separate issue
• Related:
  • Remove direct V2 engine calls from V3 mutation path #200
  • Extract MutationEngine and TableBinding abstractions #201