[Chore] Remove the legacy query engines from Measure, Stream, and Trace #14066

Summary

Delete the separate legacy query engines and make vectorized execution the only supported engine. This is the legacy-engine removal portion of parent #13998, not completion of end-to-end batch-native query processing.

Size audit and status

Classification: tracking child for an existing cleanup PR, not an automated implementation task.
Implementation: apache/skywalking-banyandb#1326.
Boundary: legacy planners, engine dispatch/fallback, and obsolete storage-result APIs across Measure, Stream, and Trace.
This tracks the existing cross-engine cleanup; it is not a new request to implement the remaining batch-native work in that PR.

Scope

• Remove row-based planners, dispatch branches, fallback execution, and obsolete storage-result code.
• Remove tests and benchmarks whose only subject was the deleted engine.
• Document removed fallback behavior and temporary compatibility flags.
• Remove the row-call-site lint guard from PR chore: remove the row-based query path from measure, stream and trace skywalking-banyandb#1326; human review is sufficient.

Completion criteria

• PR chore: remove the row-based query path from measure, stream and trace skywalking-banyandb#1326 merges after resolving its conflicts with main and preserving perf(stream): bound the filtered index-order vectorized Top-N merge skywalking-banyandb#1331's filter-first, bounded index-order merge.
• Measure, Stream, and Trace use the vectorized engine without fallback to the deleted engine.
• Explicitly disabling vectorized execution fails at startup with a clear error; omission and =true remain accepted during the documented transition.
• Relevant unit and standalone/distributed integration suites pass.
• The PR links the remaining workstreams and closes only this child, leaving [Chore] Remove row-based query from Stream, Measure and Trace #13998 open.

Verification

Startup tests cover omission, =true, and =false for the compatibility flags. Query fixtures preserve selected values, ordering, and pagination; documented behavior changes remain explicit.

go test ./pkg/query/... ./banyand/query ./banyand/measure ./banyand/stream ./banyand/trace
go test ./test/integration/standalone/query/... ./test/integration/distributed/query/...

Out of scope

Row-based processing still used internally by the vectorized engine; implementing the four follow-up workstreams; removing transition flags ahead of the documented release. The umbrella retains the final flag/adapter cleanup audit.

Parent: #13998.

Compatibility and review policy

• Preserve existing on-disk and wire formats and mixed-version compatibility. Any necessary protocol change requires a separately reviewed compatibility contract.
• Use explicit, hand-calculated expected results; do not depend on the removed row engine as the test oracle.
• Human review enforces the no-new-row-path policy. A row-call-site lint guard is not required.