Edge delta producer: guarantee delta ≤ full (min(full,delta) clamp) + decode-free / decoded-base caching

[zzylol]

What

Makes the edge sketch producer (asap-precompute-go) never emit a delta frame larger than the equivalent full frame at the same emit cadence — the delta-vs-full bandwidth invariant — and removes redundant per-window work on the emit path. Delta transmission stays OFF by default; this only changes what a delta frame looks like when enabled.

Why

A delta is only a win when it's smaller. But the sketch full frames are themselves compact (DDSketch packs a contiguous positional count array; HLL uses a sparse-packed register encoding), so a naive sparse/indexed delta can be larger than the full frame for dense (DDSketch) or low-cardinality (HLL) data. Measured on the SERF/Chimp/Elf real-world datasets, HLL single-emit deltas ran ~2.5× the full frame. The producer now bounds this.

Changes

• min(full, delta) clamp
  • DDSketch — new DeltaFromEmpty computes the per-window delta against a fresh empty base (decode-free — no deserialize of the cached base) and returns the full frame whenever the delta isn't strictly smaller.
  • HLL — ComputeDeltaAgainst returns full when the register-update delta isn't smaller than the sparse-packed full frame (corrects the prior "register deltas are never larger" comment, false at low cardinality).
• Decode-free / decoded-base caching
  • DDSketch per-window empty-base path no longer re-deserializes the (empty) cached base each window.
  • HLL/CMS/CountSketch ("always-refresh") cache a value-copy of just the count/register arrays the diff reads (clone-on-store), so the prior window's snapshot is never re-deserialized. Byte-parity tested against the serialize+decode path.
• docs — renamed the design-alternative codenames to descriptive per-window-reset (PWR) / cumulative-epoch (CE).

Testing

• Per-family clamp invariant (delta ≤ full, both clamp directions)
• Decoded-base byte-parity (HLL/CMS/CountSketch): decode-free path == serialize+decode path, bit-for-bit
• Decode-free round-trips + dense-vs-sparse DDSketch cases
• go test ./... green; edge processor builds

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