Build Filter Optimization:: Fused derive

[DhruvilK7]

Fuse Phase 2 (derive) into addRange so that each key hash is derived and banded in a single pass, eliminating the intermediate []hashResult allocation that accounted for ~53% of total build memory. The one-ahead pipeline (nextHr = derive(hashes[i+1]) before add(currHr)) preserves the existing prefetch-friendly access pattern while removing one full O(n) allocation and traversal.

benchmark                                        old ns/op     new ns/op     delta
BenchmarkBuildFilter/w=64/n=1000-11              107618        105230        -2.22%
BenchmarkBuildFilter/w=64/n=10000-11             332135        288198        -13.23%
BenchmarkBuildFilter/w=64/n=100000-11            3820237       3633888       -4.88%
BenchmarkBuildFilter/w=128/n=1000-11             157540        154127        -2.17%
BenchmarkBuildFilter/w=128/n=10000-11            698448        640460        -8.30%
BenchmarkBuildFilter/w=128/n=100000-11           6744833       6238794       -7.50%

benchmark                                        old allocs    new allocs    delta
BenchmarkBuildFilter/w=64/n=1000-11              8             7             -12.50%
BenchmarkBuildFilter/w=64/n=10000-11             8             7             -12.50%
BenchmarkBuildFilter/w=64/n=100000-11            8             7             -12.50%
BenchmarkBuildFilter/w=128/n=1000-11             9             8             -11.11%
BenchmarkBuildFilter/w=128/n=10000-11            9             8             -11.11%
BenchmarkBuildFilter/w=128/n=100000-11           9             8             -11.11%

benchmark                                        old bytes     new bytes     delta
BenchmarkBuildFilter/w=64/n=1000-11              54160         21392         -60.50%
BenchmarkBuildFilter/w=64/n=10000-11             532496        204816        -61.54%
BenchmarkBuildFilter/w=64/n=100000-11            5226640       2023568       -61.28%
BenchmarkBuildFilter/w=128/n=1000-11             60816         28048         -53.88%
BenchmarkBuildFilter/w=128/n=10000-11            622608        294928        -52.63%
BenchmarkBuildFilter/w=128/n=100000-11           5996688       2793616       -53.41%