asap_sketchlib ↔ sketchlib-go byte-format harmonization (cross-language parity)

[zzylol]

Background

PR #242 (Phase 3 step 2) added a cross-language byte-parity harness comparing envelopes produced by `asap-precompute-go` (Go runtime + sketchlib-go) vs `asap-precompute-rs` (Rust runtime + asap_sketchlib). All 5 sketches diverge byte-for-byte; tests are honestly `#[ignore]`'d in `asap-precompute-rs/tests/cross_language_parity.rs` with documented reasons.

This issue tracks the asap_sketchlib + sketchlib-go work needed to close the gap.

Per-sketch divergences

• DDSketch: `asap_sketchlib::DdSketch.store_counts` includes 128-aligned padding zeros; `sketchlib-go` emits a tight bucket array.
• KLL: `asap_sketchlib::KLL` doesn't expose `levels()` / `items()` accessors; the Rust wrapper builds `KllState.items` from a copy-on-update history vec, which diverges from sketchlib-go's compactor view.
• HLL: `asap_sketchlib::HllSketch::update` uses a different hash seed path than `sketchlib-go::HyperLogLog::Update`; registers populated differently.
• CountSketch: `asap_sketchlib` uses `twox_hash::XxHash64::oneshot(r, key)`; `sketchlib-go` uses `DeriveIndex/DeriveSign` over a seeded HashSpec table.
• CountMinSketch: same hash divergence as CountSketch.

asap_sketchlib API gaps

• No `serialize_portable()` / `serialize_proto_bytes()` helpers — wrappers in `asap-precompute-rs/src/sketches/` build the proto manually.
• No `compute_delta(prev, current, threshold)` — wrappers force `is_full=true`, bandwidth-inefficient vs Go.
• No `KLL::levels()` / `KLL::items()` accessors.
• No hash-layer compatibility shim with `sketchlib-go`'s `CanonicalHashSeed` table.

Why this matters

ADR-0002 promises bit-identical wire format across runtimes. Without harmonization, a fleet mixing Go and Rust ASAP edge agents would produce envelopes that backends cannot merge cross-source.

Scope

• Multiple PRs across `sketchlib-go` and `asap_sketchlib` (and a follow-up in this repo to un-`#[ignore]` the parity tests).
• Deferred until there's a real Rust-based edge agent consumer (Vector adapter, OTAP-Rust receiver). For now, Go-only fleets are unaffected.