Move summary kernel serialization formats out of Planner #471

Background

While splitting #462, we set the boundary that ASAPPlanner should not contain SDS or deployment-specific storage/wire formats. Each deployment picks its own formats. #462 already moves these back to ASAPQuery-backend:

• types/post_asap/semantic_definition.rs (v1 and v2)
• stored_state/{delta_apply,decoders,native}.rs
• the encode/decode version envelopes of CompiledPhysicalDag and PhysicalCandidate

Serialization inside the summary kernels was deliberately left out of #462 to keep the split scope bounded. This issue tracks it.

Scope

In crates/asap-physical-operators on the #462 branch:

• SummaryKernel in summary_kernels/traits.rs requires serialize_to_json and serialize_to_bytes.
• Every sketch kernel has its own msgpack/proto encode/decode, e.g. count_min_sketch.rs, count_sketch.rs, dd_sketch.rs, hll_sketch.rs, datasketches_kll.rs, *_with_heap.rs.
• summary_kernels/sketch_envelope.rs and the asap_sketch_codec crate.
• serialize_to_bytes / deserialize_from_bytes on KeyByLabelValues and Measurement.

This code was moved from ASAPQuery-backend/data_plane/src/precompute_engine/operators/*_accumulator.rs with only minor changes.

Observation

On the #462 branch, apart from summary_kernels/ itself and stored_state/ (which is moving back to Backend), no Planner code calls these serializers. Build/merge/readout work on in-memory state. So once stored_state moves back, the byte formats are probably used only by Backend.

Questions

1. Can the serialization methods be dropped from SummaryKernel, keeping only the in-memory state API (build/merge/readout)?
2. Should msgpack/proto encoding and asap_sketch_codec move to Backend, next to the SDS wire decoding in sketch_db?
3. Does Backend need an accessor to read or rebuild kernel state (for example, exposing the underlying asap_sketchlib structure)? The goal is to avoid adding a new abstraction layer.

Non-goals

• Changing sketch algorithms, which stay in asap_sketchlib.
• Blocking feat: separate physical planning from deployment and share DAG execution #462 or its split PRs.