[Chore] Merge Stream results as batches across groups and data nodes

[hanahmily]

Summary

Remove intermediate Element materialization from Stream result merging and deduplication. Follow-up to #14066 under #13998.

Size audit and dependencies

Classification: tracking parent, not an executable leaf or automated implementation task.
Boundaries: local multi-group dispatch using MergeGroupElements, and the distributed coordinator that currently decodes frames into Elements before merging.
These are separate production cutovers and must become separate executable tasks.
Blocked by: #14066 via BanyanDB PR apache/skywalking-banyandb#1326.
Related dependency: #14067. Coordinate with that workstream for full filtered/hidden-tag coverage; basic frame-input coordinator merging can be sliced independently. Do not start a child whose required projection boundary is not yet merged.

Requirements

R1. Merge and deduplicate batches directly for both local multi-group and distributed callers.
R2. Preserve ordering, selected duplicate values, and global offset/limit behavior. Retain internal sort keys even when clients do not project ordering tags.
R3. Accept frame and protobuf node responses for tracing and mixed-version compatibility. Convert protobuf input at the compatibility boundary, rather than forcing frame inputs back into Elements.
R4. Materialize final Elements only where the client output requires them; release batch resources on completion, cancellation, and malformed responses.

Concrete oracle

Source 1: A(key=1), B(key=3)
Source 2: A(key=1), C(key=2)

The two copies of A are identical. Ascending order with offset 1 and limit 2 returns C, B. Exercise the same oracle across batches, across local groups, and across data-node replies. Specify duplicate-version and equal-key fixtures separately when defining each executable child; do not invent a new tie rule as a side effect of the cutover.

Completion criteria and RED-test direction

• Real local and distributed callers use batch merge/deduplication, including cross-batch duplicates and pagination.
• Mixed frame/protobuf replies preserve the oracle and tracing information.
• Path-sensitive tests fail if frame inputs are materialized into Elements before merging. Output assertions alone are not RED because the current row merge can return the same answers.
• Cancellation, malformed replies, and batch release are covered in each caller cutover.
• Benchmark comparisons quantify allocation changes; no new wire format or unproved bounded-memory guarantee is required.

Relevant code: banyand/query/processor.go, pkg/query/logical/stream/stream_plan_merge.go, pkg/query/logical/stream/stream_plan_distributed.go, and pkg/query/vectorized/stream.

go test ./banyand/query ./pkg/query/logical/stream ./pkg/query/vectorized/stream/...
go test ./test/integration/standalone/query/... ./test/integration/distributed/query/...

Out of scope

Changing filter-first semantics, timestamp under-fill behavior, or wire formats; implementing Stream filtering/projection itself (separate workstream).

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.