SPANN SelectHead: opt-in parallel BKT build

[fastio]

Motivation

For billion-scale datasets, BKT construction in SelectHead dominates index
build time. The existing build is a serial node schedule where only
KmeansAssign inside a single node is data-parallel. That works well near the
root (few large nodes) but leaves most cores idle once the tree fans out into
many small subtrees.

Design

Two-phase parallel build, gated by a new [SelectHead] ParallelBuildBKT option
(default false, i.e. the existing serial path is untouched unless opted in):

• Phase 1 (top of the tree): serial node schedule, each node clustered with
  the existing intra-node data-parallel KmeansAssign using all threads. The
  few large nodes near the root are exactly where data parallelism is
  effective.
• Phase 2 (bottom of the tree): once a subtree's size drops below a cutoff
  (max(BKTLeafSize * 4, N / (threads * 16) + 1)), it is deferred and later
  built whole by a single worker. Workers pull deferred subtrees from a shared
  queue (fetch_add), each with its own KmeansArgs and _TH = 1, so the
  many small nodes are processed without per-node thread spawning.

All workers share the single N-sized label buffer: sibling nodes own disjoint
[first, last] ranges of localindices (including the center slot at last),
so writes never overlap.

Concurrency changes

• Utils::rand now uses a thread_local std::mt19937. std::rand() shares
  hidden global state and races under concurrent clustering. Note this also
  affects the serial path: previously std::rand() without srand produced
  the same sequence on every run; builds are now seeded fro
  std::random_device and are no longer deterministic run-to-run. Quality is
  equivalent; verification should compare recall, not bits.
• Structural writes to m_pTreeRoots are guarded by a mutex; writes to
  m_pSampleCenterMap by another. The clustering itself is
• KmeansAssign gains a _TH == 1 serial fast path so Phase 2 workers do not
  spawn/join a thread per node.

Compatibility

• Default off; with the option unset the build takes the or
• Parallel builds are not bit-identical to serial ones (RNG, scheduling, and
  floating-point summation order differ); tree quality is e

Also included

A second commit with portability fixes needed to consume SP
under a strict libc++ toolchain at C++23:

• The non-TBB ConcurrentQueue/ConcurrentSet/ConcurrentMap fallbacks now
  expose the TBB API surface that ExtraFileController/Ex already use (unsafe_size, empty, unsafe_begin/end, value_type`,
  range iteration), so the no-TBB configuration compiles ag
• LoggerHolder always takes the std::atomic_load/std::atomic_store path:
  the C++20 std::atomic<std::shared_ptr<Logger>> speciali
  by libc++ and the pre-C++20 path is valid under both standards.
• Pass .c_str() to a SPTAGLIB_LOG varargs call in Appr (passing std::stringthrough...` is undefined behavior).
• Drop an unused <omp.h> include from TxtReader.cpp.

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