[tsenart]

The ItemsSketch.Update method and its compression helpers were
allocating unnecessarily, causing significant GC pressure in
high-throughput scenarios.

Three changes:

1. Store minItem/maxItem by value instead of pointer. The previous
   implementation used *C pointers and assigned &item on each update,
   causing the item parameter to escape to heap. Now we store values
   directly and use a hasMinMax bool to track initialization state.

2. In compressWhileUpdatingSketch, work on s.items directly instead
   of calling GetTotalItemsArray() which copies the entire items slice.
   All operations (sort, halve, merge, shift) modify the array in
   place, so the copy was unnecessary.

3. In addEmptyTopLevelToCompletelyFullSketch, use s.levels and
   s.items directly instead of getLevelsArray() and GetTotalItemsArray().
   The copies were only used to read current state before growing.

Benchmarks

On a 500k item workload:

Metric	Before	After	Delta
time/op (KeyFuncNone)	89ms	34ms	-62%
time/op (KeyFuncZorder)	251ms	187ms	-26%
alloc/op (KeyFuncNone)	472MB	4.4MB	-99%
alloc/op (KeyFuncZorder)	611MB	143MB	-77%
allocs/op (KeyFuncNone)	518,979	210	-99.96%
allocs/op (KeyFuncZorder)	1,019,840	501,097	-51%

All existing tests pass.

[freakyzoidberg]

Nice catch - Thanks !