perf(algo): pre-size heap in MergeSortedPacked#9714
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Build the heap slice and call heap.Init (O(n)) instead of n×heap.Push, which
boxes each elem into an interface{} (one heap alloc per input list).
BenchmarkMergeSortedPacked (1000 lists × 100 uids):
allocs/op 104.1k -> 103.0k (-0.97%, p=0.002)
B/op 21.34Mi -> 21.22Mi (-0.58%, p=0.002)
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Build the heap slice and call
heap.Init(O(n)) instead of n×heap.Push, which boxes eacheleminto an interface{} (one heap alloc per input list).BenchmarkMergeSortedPacked(1000 lists × 100 uids): allocs/op 104.1k→103.0k (-0.97%), B/op -0.58%.Independent change; branched from
4b9b399d(no dependency on the other PRs in this set). Verified: package unit tests pass + Go benchmark (benchstat, p<0.01). Numbers are single-thread microbenchmarks; not yet run through the Docker integration suite.