refactor(query-engine): remove obsolete heap top-k APIs and paths #685

Parent

#678

What to build

Consolidate the CountMinSketchWithHeap API and top-k execution path. The current implementation contains a compatibility re-export, an unimplemented deserialization method, multiple serialization paths, a dead formatting pipeline, and stale comments.

Coordinate with #673 for serialization behavior and #670 for the typed aggregation representation.

Acceptance criteria

• One supported heap serialization contract is clearly defined.
• Unimplemented or dead public APIs are removed, implemented, or explicitly deprecated.
• The heap-item public API has a stable name and ownership.
• Top-k sorting, formatting, and truncation have one authoritative execution path.
• Stale top-k comments and documentation are corrected.
• Existing compatibility requirements are covered by tests.

Blocked by

• Fail loudly when accumulator serialization fails #673 — serialization failures must be handled before removing or changing serialization APIs.
• refactor(asap-types): model aggregation kinds and subtypes with typed enums #670 — typed aggregation representation may define the final heap API boundary.

Concrete examples

The current public surface contains patterns like:

pub use asap_sketchlib::CmsHeapItem as HeapItemReexport;
pub fn deserialize_from_bytes(...) -> Result<Self, Error> {
 Err("... not implemented".into())
}

The query engine also retains a dead-code-marked formatter while the active range pipeline performs top-k sorting and truncation elsewhere. In addition, SQL code still contains a comment equivalent to:

// SQL doesn't support topk limiting yet

even though the execution path supports heap-based top-k limiting.

The cleanup should leave one documented serialization contract and one authoritative top-k execution path.