feat(countsketch-topk): add portable CountSketchWithHeap wire type

[zzylol]

Summary

Fixes a real correctness gap surfaced while implementing ASAPQuery-backend's SummaryExecutor (data_plane, see ASAPQuery-backend#411): the only heap-bearing frequency wire type this crate exposed was CountMinSketchWithHeap (portable::countminsketch_topk). data_plane's query-side reducer reused that same type for both CmsWithHeap and CountSketchWithHeap sids — meaning every CountSketchWithHeap query was being read out with CMS's min-over-rows estimator instead of Count Sketch's own median-of-signed-rows estimator. The sketches::CSHeap computational type (median estimator, its own doc already says "mirrors CMSHeap but with Count Sketch") already existed and was already correct — there was just no portable/wire-format wrapper for it, unlike every other sketch family in this crate.

Adds message_pack_format::portable::countsketch_topk::CountSketchWithHeap, structurally mirroring CountMinSketchWithHeap (same wire shape: {sketch: {sketch, row_num, col_num}, topk_heap: [{key, value}], heap_size}) but backed by CSHeap instead of CMSHeap.

Not a new cross-language protocol surface. The wire shape is unchanged from what CountMinSketchWithHeap already produces/consumes for this data (I copied its wire struct verbatim) — this is a new Rust-side reader/builder for the same bytes with correct math, not a new format sketchlib-go needs to learn. No Go-side changes needed.

Also adds #[derive(Clone)] to sketches::CSHeap (CMSHeap already had it; CSHeap was missing it — needed for CountSketchWithHeap::merge_refs's structural clone; both of CSHeap's own fields, Count and HHHeap, are already Clone, so this was just an oversight, not a design constraint).

Test plan

• cargo test message_pack_format::portable::countsketch_topk — 8 new tests, all passing: creation, empty query, merge (+ dimension mismatch), msgpack round-trip, aggregate_topk (+ empty), and test_median_estimator_differs_from_cms_style_min — the actual point of this addition, proving the median estimator behaves differently from a CMS-style min estimator on a skewed insert pattern, not just sharing a struct shape.
• cargo test (full workspace) — 466 unit + 5 asapv1_golden + 12 msgpack_compat (5 ignored) + 4 sketches_go_parity_probe + 1 xtest_consumer + 19 doctests, all passing.
• cargo clippy --all-targets --all-features -- -D warnings — clean.
• cargo fmt --check — clean.

🤖 Generated with Claude Code