gh-NNNN: Use _PyWuffs_strtod in float string parsing, replacing _Py_dg_strtod

Replace all calls to _Py_dg_strtod() with _PyWuffs_strtod() from the
newly-vendored Wuffs float-parsing adapter:

* Python/pystrtod.c: _PyOS_ascii_strtod() now calls _PyWuffs_strtod()
  (the Wuffs Eisel-Lemire + HPD parser) instead of Gay's _Py_dg_strtod().
* Objects/floatobject.c: double_round() uses _PyWuffs_strtod() for the
  string-to-double step when converting back from a rounded decimal string.

_PyWuffs_strtod() is a correctly-rounded, Bigint-free, platform-strtod-free
replacement. It uses the Eisel-Lemire fast path for ~99.9999% of inputs and
falls back to the High Precision Decimal (HPD) algorithm for the remainder.
Both paths are self-contained within the vendored Wuffs header.

After this change _Py_dg_strtod() has no callers outside dtoa.c itself
and will be removed in the next commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: eendebakpt

Objects/floatobject.c

@@ -17,6 +17,9 @@
 #include "pycore_stackref.h"      // PyStackRef_AsPyObjectBorrow()
 #include "pycore_structseq.h"     // _PyStructSequence_FiniBuiltin()
 #include "pycore_tuple.h"         // _PyTuple_FromPair
+#if _PY_SHORT_FLOAT_REPR == 1
+#include "../Python/_ryu/pystrtod_wuffs.h" // _PyWuffs_strtod
+#endif
 
 #include <float.h>                // DBL_MAX
 #include <stdlib.h>               // strtol()
@@ -939,7 +942,7 @@ double_round(double x, int ndigits) {
     /* and convert the resulting string back to a double */
     errno = 0;
     _Py_SET_53BIT_PRECISION_START;
-    rounded = _Py_dg_strtod(mybuf, NULL);
+    rounded = _PyWuffs_strtod(mybuf, NULL);
     _Py_SET_53BIT_PRECISION_END;
     if (errno == ERANGE && fabs(rounded) >= 1.)
         PyErr_SetString(PyExc_OverflowError,

Python/pystrtod.c

@@ -1,9 +1,13 @@
 /* -*- Mode: C; c-file-style: "python" -*- */
 
 #include <Python.h>
-#include "pycore_dtoa.h"          // _Py_dg_strtod()
+#include "pycore_dtoa.h"          // _Py_dg_dtoa(), _Py_dg_freedtoa()
 #include "pycore_pymath.h"        // _PY_SHORT_FLOAT_REPR
 
+#if _PY_SHORT_FLOAT_REPR == 1
+#include "_ryu/pystrtod_wuffs.h"  // _PyWuffs_strtod()
+#endif
+
 #include <locale.h>               // localeconv()
 
 /* Case-insensitive string match used for nan and inf detection; t should be
@@ -101,7 +105,7 @@ _PyOS_ascii_strtod(const char *nptr, char **endptr)
     errno = 0;
 
     _Py_SET_53BIT_PRECISION_START;
-    result = _Py_dg_strtod(nptr, endptr);
+    result = _PyWuffs_strtod(nptr, endptr);
     _Py_SET_53BIT_PRECISION_END;
 
     if (*endptr == nptr)