fix: REQ sketch loses min/max when merging into a new sketch

[jaideeppyne]

req.NewSketch() leaves minItem/maxItem at Go's zero value, but Merge and Reset both use NaN as the "unset" sentinel. So merging into a freshly created sketch computes min(0, other.minItem) and max(0, other.maxItem), and the 0 wins whenever the stream doesn't straddle it.

src, _ := req.NewSketch()
for i := 1001; i <= 1100; i++ { src.Update(float32(i)) }

dst, _ := req.NewSketch()
dst.Merge(src)

dst.MinItem()   // 0, want 1001
dst.Quantile(0) // 0, want 1001
dst.Rank(500)   // 0.01, want 0

It's not just the getters. The bogus 0 is reinserted into the sorted view as a retained quantile, so Quantile, Rank, CDF and PMF all shift, and it survives MarshalBinary + Decode. Chained merges into an empty accumulator, which is the usual map-reduce shape, hit it too. NewSketch() then Reset() then Merge works fine, which is the tell.

ReqSketch.java initialises the fields at declaration:

private float minItem = Float.NaN;
private float maxItem = Float.NaN;

Go has no field initialisers so the port dropped it. I set them in NewSketch instead. That covers every construction site: the decoder's empty and raw-items paths both go through NewSketch, and the exact and estimation paths read min/max out of the buffer. Update guards on IsEmpty() so it was never affected, and every reader of the fields (MinItem, MaxItem, refreshSortedView, the estimation-format encoder) is already behind an emptiness check, so no path can now observe the NaN.

How I found it: I built an invariant harness over the quantile families asserting that min/max after merging into a fresh sketch equals min/max after merging into a reset sketch, and equals the source's. 5 value patterns across REQ, KLL and t-digest. KLL and t-digest are clean; REQ fails 4 of 5. The one that passes is the case whose data straddles zero, which is why the existing Merge Multiple test misses it. I checked the intended semantics against ReqSketch.java rather than guessing at it. The same harness also ran 200-trial randomised sweeps per family for quantile monotonicity in rank, CDF within [0,1] and non-decreasing, PMF summing to 1, and serialize/deserialize identity; nothing else came out of it, and I confirmed tdigest.Quantile matches tdigest_impl.hpp line for line.

7 subtests added in TestSketchMergeIntoEmptyMinMax. All 7 fail on main with only the test file applied, all 7 pass with the two-line change. make lint-check and go test -race ./req/ are clean and the full suite is green.

Written with AI assistance (Claude). I reviewed the change and ran everything above myself.

[proost]

Nice Catch! Thank you!