sketches: structural Clone (KLL, CMS-heap) + KLL direct bit-exact reconstruction

[zzylol]

Summary

Avoid serialize→deserialize / extract→rebuild round-trips when copying or
decoding the portable sketches.

KLL

1. Structural KllSketch::clone. clone() previously did a full msgpack
   serialize → deserialize round-trip. The backing KLL<f64> already derives
   Clone, so this clones it structurally instead — same sketch, no rmp
   encode/decode, far fewer allocations.

2. KLL::from_portable_state — direct reconstruction. Rebuilds a sketch
   directly from its portable wire form (k + level-ordered items + the
   levels[] boundary array) by placing items into the internal buffer at the
   correct offset and fixing up the level boundaries, instead of replaying every
   retained item through update(). This is bit-exact (identical quantiles
   to the source — the replay path was a lossy statistical reconstruction) and
   several× faster. Exposed via KllSketch::from_portable_state(...) for
   proto/envelope decoders; the empty-sketch case is handled.

CMS-with-heap

3. Structural CountMinSketchWithHeap::clone. CMSHeap's fields
   (CountMin, HHHeap) both derive Clone, so CMSHeap now derives Clone
   and the wrapper clones the backend structurally instead of extracting the
   count matrix + top-k heap to wire form and rebuilding it on every clone.

Test

• from_portable_state_reproduces_source_exactly asserts the reconstructed KLL
  sketch's quantiles match the source exactly across [0.0 .. 1.0], plus the
  empty-sketch round-trip.
• CMS-with-heap clone/round-trip is covered by the existing suite
  (count_min_sketch_with_heap_round_trip, count_min_with_heap_wire_shape).

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