Reproducible floating-point summations

Summary:
Introduce ReproducibleFloatingAccumulator. Header-only C++ library for reproducible (order-independent) floating-point summation using binned floating-point arithmetic, adapted from ReproBLAS v2.1.0.

Key features of the accumulator:
- Reproducible summation independent of summation order
- Accuracy at least as good as conventional summation, and tunable via FOLD
- Handles overflow, underflow, NaN, and infinity reproducibly
- Single read-only pass over summands; minimal memory (2*FOLD floats - usually 6 since FOLD=3 is sufficient for most purposes)

Public interface:
- Default, copy, and implicit scalar construction
- operator+=/-= for scalars and accumulators
- Binary operator+/- (including scalar-accumulator mixed expressions)
- operator== for bitwise equality
- Explicit conversion to native float via operator ftype() and value()
- Batch add() via iterator pairs, C++20 ranges, or single values
- Manual unsafe_add/renorm path for performance-critical loops
- Error bound computation
- fmt::formatter specialization (always available via fmt/format.h)
- static_assert(FOLD >= 2) to catch invalid instantiation

# Example

```
ReproducibleAccumulator<double> rfa;
for (const auto& x : kDoubleData) {
  rfa += x;
}
rfa.value(); // Same result irrespective of ordering of kDoubleData
```

# Benchmark
```
============================================================================
[...]/ReproducibleAccumulatorBenchmark.cpp     relative  time/iter   iters/s
============================================================================
SumDouble_OneAtATime_10                                    50.69ns    19.73M
SumDouble_BatchAdd_10                           55.744%    90.93ns    11.00M
----------------------------------------------------------------------------
SumDouble_OneAtATime_1000                                   4.11us   243.32K
SumDouble_BatchAdd_1000                         132.29%     3.11us   321.88K
----------------------------------------------------------------------------
SumDouble_OneAtATime_1M                                     4.11ms    243.12
SumDouble_BatchAdd_1M                           137.09%     3.00ms    333.30
----------------------------------------------------------------------------
SumFloat_OneAtATime_1M                                      4.14ms    241.69
SumFloat_BatchAdd_1M                            137.35%     3.01ms    331.96
----------------------------------------------------------------------------
SumDouble_NaiveBaseline_1M                                  1.03ms    975.29
SumFloat_NaiveBaseline_1M                                   1.02ms    975.89
----------------------------------------------------------------------------
SumDouble_Kahan_1M                                         13.31ms     75.11
SumFloat_Kahan_1M                                          13.32ms     75.10
----------------------------------------------------------------------------
SumDouble_LongDouble_1M                                     2.39ms    418.41
SumFloat_LongDouble_1M                                      2.39ms    418.72
```

Reviewed By: yfeldblum

Differential Revision: D97780214

fbshipit-source-id: 07399b5f905f3a021252560ad80df47fe53db21a

Author: r-barnes

folly/CMakeLists.txt

@@ -1419,6 +1419,7 @@ add_subdirectory(io)
 add_subdirectory(json)
 add_subdirectory(lang)
 add_subdirectory(logging)
+add_subdirectory(math)
 add_subdirectory(memory)
 add_subdirectory(net)
 add_subdirectory(observer)

folly/math/BUCK

@@ -0,0 +1,13 @@
+load("@fbsource//tools/build_defs/dirsync:fb_dirsync_cpp_library.bzl", "fb_dirsync_cpp_library")
+
+oncall("fbcode_entropy_wardens_folly")
+
+fb_dirsync_cpp_library(
+    name = "reproducible_accumulator",
+    headers = ["ReproducibleAccumulator.h"],
+    use_raw_headers = True,
+    exported_deps = [
+        "fbsource//third-party/fmt:fmt",
+        "//folly:constexpr_math",
+    ],
+)

folly/math/CMakeLists.txt

@@ -0,0 +1,23 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# @generated by folly/facebook/generate_cmake.py
+
+folly_add_library(
+  NAME reproducible_accumulator
+  HEADERS
+    ReproducibleAccumulator.h
+  EXPORTED_DEPS
+    folly_constexpr_math
+)