[gouyt13]

Summary

Consolidate scalar quantization type selection with if constexpr and share rounding helper implementations while preserving their entry points. Default ordinary rotator copying, simplify RabitqConfig special members, name core buffer sizes with structured bindings, and reuse class-qualified rotator serialization sizes without changing subclass dispatch behavior.

Remove the unused rabitqlib::Array, its dedicated memory::Allocator, installation entry, and three Array-only tests. This removes the installed rabitqlib/utils/array.hpp header and those C++ types; external consumers using them must migrate. RowMajorArray, the stable core v1 API, saved-index formats, and dependencies are unchanged.

Testing

• Combined portable Release C++ unit/reference suite: 367 passed; the OpenMP binding test skipped in the default run passed separately with binding enabled.
• Frozen core v1 consumer and dispatch/scalar zero-allocation checks passed. Fresh installation and all three installed-consumer CTests passed.
• Focused checks covered helper compatibility, config copy/move traits, rotator copy/self-assignment, and buffer/stream round trips including subclasses overriding serialized size. GCC 12 / Clang 23 scalar quantization code-generation probes matched the original instructions and relocations.
• Full configured clang-tidy, formatting, and git diff --check passed.
• No local Windows, macOS, ARM64, or Python wheel rerun for the combined changes; those remain CI coverage.