Add stable LSD radix sort library

Summary:
Add a generalized stable radix sort implementation to folly/algorithm. This extracts and generalizes techniques from the AdInfoStorage radix sort implementation into a reusable library.

The algorithm was initially ideated/implemented by konstantin1, so the credit should go to him for coming up with it.

Key features:
- Stable O(n) LSD (Least Significant Digit) radix sort
- Handles unsigned/signed integers and floating-point types
- Custom key extractors for sorting by struct members
- Ascending and descending order variants
- IEEE 754 to sortable key transformation for floats/doubles
- Alternating buffer pattern to minimize copying
- Falls back to insertion-sort for small inputs (<16 elements)

I ended up doing it because marksantaniello requested it :)
It was `claude`d all the way....

Reviewed By: yfeldblum

Differential Revision: D91785878

fbshipit-source-id: a85fc6a4282a120ef84c7fafff3fbab9d8f300d7

Author: Anatoly Akkerman

folly/algorithm/BUCK

@@ -15,3 +15,13 @@ fb_dirsync_cpp_library(
         "//folly/lang:builtin",
     ],
 )
+
+fb_dirsync_cpp_library(
+    name = "stable_radix_sort",
+    headers = ["StableRadixSort.h"],
+    use_raw_headers = True,
+    xplat_impl = folly_xplat_library,
+    exported_deps = [
+        "//folly:traits",
+    ],
+)

folly/algorithm/CMakeLists.txt

@@ -22,4 +22,12 @@ folly_add_library(
     folly_lang_builtin
 )
 
+folly_add_library(
+  NAME stable_radix_sort
+  HEADERS
+    StableRadixSort.h
+  EXPORTED_DEPS
+    folly_traits
+)
+
 add_subdirectory(simd)