[memory]: Memory breakdown reporting and estimation functionality have to handle Hugepage allocations rounded to 2MB

[rfsaliev]

This is the fallout from IndexBuilder storing a live handle rather than a builder: the estimator has no way to know which allocator was configured, so it has to hardcode one. The result is that svs_index_builder_estimate_memory_dynamic is wrong for SVS_ALLOCATOR_KIND_HUGE_PAGE (2 MiB rounding is unaccounted for) and wrong for any custom allocator. dispatch_vamana_memory_estimate (~line 159 of dispatcher_vamana.cpp, unchanged here) is allocator-blind entirely — it still uses SimpleDataBuilder<index_type>{}.

Note that if the motivation for defaulting to lib::Allocator was to make the estimate line up with within_1pct(tracker.live_bytes, memory_usage) in the new tests, that's fixing the estimator's inaccuracy by degrading the runtime. Threading the kind through instead fixes both.

Originally posted by @ethanglaser in #380