Add weighted update to kll_sketch

[shanielh]

Adds kll_sketch::update(item, weight), equivalent to calling update(item) weight times. This mirrors update(item, weight) on the KLL sketches in datasketches-java.

Behavior

• If weight is smaller than the free space in level zero, the item is inserted weight times as plain updates. No compaction can happen on this path.
• Otherwise, the update builds an exact sketch of the weighted item and merges it in. Since an item at level h counts as 2^h, the exact sketch holds one copy of the item at each level whose bit is set in weight. Bits above level 60 fold into weight >> 60 copies at level 60, because level capacities are only defined up to 61 levels. This path costs one merge with a sketch of at most 75 items, however large the weight.
• A weight of zero throws std::invalid_argument, the same as in Java. NaN items are ignored, as in update(item).
• Serialization is unchanged, so a sketch built with weighted updates stays readable by every other implementation.

Fix included

The first commit fixes an infinite loop in kll_helper::floor_of_log2_of_fraction. Once numer >= 2^63, doubling denom overflows to zero before it exceeds numer. ub_on_num_levels(n) calls this function with the merged stream weight, so any merge that reaches n >= 2^63 used to hang. A weighted update makes that weight easy to reach. The fix compares denom against numer / 2, which gives the same result without the overflow. The fix has its own unit cases.

Tests

New sections in kll_sketch_test.cpp:

• zero weight throws
• NaN is ignored
• matches repeated updates exactly in exact mode
• a single item with a 2^40 + 12345 weight
• UINT64_MAX - 1 weight, with a serialization round trip
• large weights merged into a full sketch
• 10,000 weighted items in estimation mode, ranks within the k=200 error bound, with a serialization round trip
• strings

The kll test suite passes on clang (macOS arm64) in a plain debug build and with -fsanitize=address,undefined.

Companion PR for datasketches-rust: apache/datasketches-rust#293

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