refactor: inline backend #ifdef into elementwise.h / linalg.h, delete shim files

Delete numpy/detail/math_backend.h and numpy/detail/linalg_backend.h — these
one-liner shim files only forwarded a single #ifdef NUMPYCPP_STD_ONLY block.
Inline the selection directly in elementwise.h and linalg.h:

  elementwise.h: NUMPYCPP_STD_ONLY → detail/std_math_backend.h
                 default           → detail/npy_math_float.h + detail/svml_bridge.h

  linalg.h:      NUMPYCPP_STD_ONLY → detail/std_linalg_backend.h
                 default           → detail/blas_bridge.h

Build verified: both bit-exact and std targets compile and all 917 tests pass.

Author: peng.li24

numpy/detail/linalg_backend.h

@@ -37,16 +37,23 @@
 #include <algorithm>
 
 // ── Internal detail headers ──────────────────────────────────────────────────
-// math_backend.h selects the math implementation at compile time:
-//   NUMPYCPP_STD_ONLY not set → svml_bridge.h (bit-exact, dlsym + SVML)
-//   NUMPYCPP_STD_ONLY     set → std_math_backend.h (pure <cmath>, perf-first)
+// Backend selected at compile time:
+//   NUMPYCPP_STD_ONLY not defined (default):
+//     svml_bridge.h + npy_math_float.h — bit-exact (dlsym + SVML + AVX-512)
+//   NUMPYCPP_STD_ONLY defined:
+//     std_math_backend.h — pure <cmath>, performance-first, no dlsym
 // avx512_loops.h provides AVX-512 specialisations; skipped in STD_ONLY mode.
 // Both require NUMPYCPP_INTERNAL_INCLUDE; we manage that here.
 #ifndef NUMPYCPP_INTERNAL_INCLUDE
 #  define NUMPYCPP_INTERNAL_INCLUDE
 #  define _NUMPYCPP_EW_OWNS_GUARD
 #endif
-#include "detail/math_backend.h"
+#ifdef NUMPYCPP_STD_ONLY
+#  include "detail/std_math_backend.h"
+#else
+#  include "detail/npy_math_float.h"
+#  include "detail/svml_bridge.h"
+#endif
 #include "detail/macros.h"   // NUMPY_UNROLL4
 
 namespace numpy {

numpy/detail/math_backend.h

@@ -29,14 +29,20 @@
 #endif
 
 // ── Internal detail headers ──────────────────────────────────────────────────
-// linalg_backend.h selects the linalg implementation at compile time:
-//   NUMPYCPP_STD_ONLY not set → blas_bridge.h (bit-exact, OpenBLAS ILP64)
-//   NUMPYCPP_STD_ONLY     set → std_linalg_backend.h (pure loops, perf-first)
+// Backend selected at compile time:
+//   NUMPYCPP_STD_ONLY not defined (default):
+//     blas_bridge.h — bit-exact (OpenBLAS ILP64, dlsym)
+//   NUMPYCPP_STD_ONLY defined:
+//     std_linalg_backend.h — pure C++ loops, performance-first
 #ifndef NUMPYCPP_INTERNAL_INCLUDE
 #  define NUMPYCPP_INTERNAL_INCLUDE
 #  define _NUMPYCPP_LINALG_OWNS_GUARD
 #endif
-#include "detail/linalg_backend.h"
+#ifdef NUMPYCPP_STD_ONLY
+#  include "detail/std_linalg_backend.h"
+#else
+#  include "detail/blas_bridge.h"
+#endif
 #ifdef _NUMPYCPP_LINALG_OWNS_GUARD
 #  undef NUMPYCPP_INTERNAL_INCLUDE
 #  undef _NUMPYCPP_LINALG_OWNS_GUARD

numpy/elementwise.h

@@ -1,42 +0,0 @@
-@PACKAGE_INIT@
-
-# numpycpp -- header-only C++ numpy API library.
-#
-# Backend selection (set BEFORE find_package, or via cmake -D):
-#   cmake -DNUMPYCPP_STD_ONLY=OFF ..   # bitexact (default): SVML + OpenBLAS via dlsym
-#   cmake -DNUMPYCPP_STD_ONLY=ON  ..   # std: pure <cmath> + C++ loops, no dlsym
-#
-# The flag is propagated automatically to every target that links numpycpp:
-#   bitexact  → INTERFACE_LINK_LIBRARIES dl
-#   std       → INTERFACE_COMPILE_DEFINITIONS NUMPYCPP_STD_ONLY
-
-# Non-namespaced + namespaced alias.
-# pybind11_add_module users: if the :: target is lost during generation, use
-#   target_include_directories(mymod PRIVATE ${numpycpp_INCLUDE_DIRS})
-add_library(numpycpp INTERFACE IMPORTED)
-
-set_target_properties(numpycpp PROPERTIES
-    IMPORTED_GLOBAL TRUE
-    INTERFACE_COMPILE_FEATURES "cxx_std_17"
-    INTERFACE_INCLUDE_DIRECTORIES "${PACKAGE_PREFIX_DIR}/include/numpycpp"
-)
-
-add_library(numpycpp::numpycpp ALIAS numpycpp)
-
-# Propagate the correct backend flags to every consumer automatically.
-if(NUMPYCPP_STD_ONLY)
-    # std backend: no external dependencies
-    set_property(TARGET numpycpp APPEND PROPERTY
-        INTERFACE_COMPILE_DEFINITIONS NUMPYCPP_STD_ONLY)
-    message(STATUS "numpycpp: std backend (pure <cmath>, no dlsym)")
-else()
-    # bitexact backend (default): libdl needed for dlsym -> numpy .so at runtime
-    set_property(TARGET numpycpp APPEND PROPERTY
-        INTERFACE_LINK_LIBRARIES dl)
-    message(STATUS "numpycpp: bitexact backend (SVML + OpenBLAS via dlsym, -ldl)")
-endif()
-
-# Fallback variables (pybind11_add_module target-loss workaround)
-set(numpycpp_INCLUDE_DIRS "${PACKAGE_PREFIX_DIR}/include/numpycpp")
-
-check_required_components(numpycpp)