feat(cs): ASAPv1 wire payload for Count Sketch

[GordonYuanyc]

What

Self-describing ASAPv1 wire serialization for Count Sketch → kind_id 0x04 0x00, following the Count-Min pattern from #65 and the KLL pattern from #75.

serialize_to_bytes / deserialize_from_bytes now emit/read the ASAPv1 envelope (replacing the old plain-rmp form), authored in a per-sketch wire submodule (src/sketches/countsketch/wire.rs).

Design decision (docs/asapv1_wire_format.md §3.4 + Q-CS)

Count Sketch mirrors Count-Min's metadata and [counts] payload with exactly one key removed: no counter_type.

CountSketchCounter requires Neg + From<i32>, so counters must be signed and negatable — which leaves i64 as the only wire-eligible type (i32 widens, i128 is not a wire type, f64 doesn't qualify at all). The kind_id already implies it, and a metadata key with a single legal value describes nothing. The variance from Q-CMS is deliberate: the two sketches are not forced symmetric where their type domains differ.

Everything else is identical to §3.2 — hash-spec group derived from the hasher's HashProfile, then rows / cols / mode; payload is the same 1-element positional array [counts], packed row-major. Cells are signed (the sketch adds ±weight), so a decoder must not assume monotonicity.

API narrowing

The wire methods are implemented only for the wire-eligible config (Vector2D<i64> storage, fast/regular mode, a profiled hasher), so the old generic-over-all-storage rmp helpers are gone and MessagePackCodec is narrowed to match. Exotic counters (i32/i128) and non-Vector2D storage must be converted first — same cut CMS made in #65. Decode is a hard cut with no legacy-rmp fallback, consistent with every other sketch.

Fail-closed decode

kind_id check; metadata equality check (hash spec + mode); non-zero dimensions (a crafted cols == 0 would otherwise panic in Vector2D::from_fn via ilog2); and a counts-length check that runs before the allocation, so crafted dimensions cannot drive a huge reserve.

Tests

Nine new tests in the wire module: round-trip; negative-counter round-trip; custom-hasher profile self-describing + cross-rejection; mode in metadata + cross-mode rejection; foreign kind_id (CMS bytes) rejection; zero dimension; dimension/length mismatch; unknown metadata key; and one asserting that adding counter_type back is rejected.

Golden byte-vectors cs_i64_regular_2x4 / cs_i64_fast_2x4 hold the same matrix and differ only by mode, so the pair also pins that mode reaches the bytes. Their values sweep the msgpack integer widths in both directions — negative fixint / int8 / int16 / int32 alongside positive fixint / uint8 / uint32 — since Count Sketch cells are signed:

9198 00 7f cc80 ce00010000 ff d0df d18000 d280000000
     0  127  u8     u32    -1  i8    i16      i32

Full suite green (491 + 9 golden); clippy --all-targets clean; fmt --check clean; cargo doc clean under CI's -D warnings.

Not in this PR

• sketchlib-go must mirror 0x04 0x00 and copy the two cs_*.hex fixtures. Deliberately out of scope here (same standing item as KLL's 0x06 0x00/0x06 0x01 from feat(kll): ASAPv1 wire payload for KLL and KLLDynamic #75).
• 0x0a 0x00 Count-Sketch-with-heap (CSHeap) is a separate kind_id and untouched.

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