gh-NNNN: Route string→float through _Py_fast_float_strtod

Swap the two in-tree _Py_dg_strtod call sites (pystrtod.c's
_PyOS_ascii_strtod and floatobject.c's double_round) for
_Py_fast_float_strtod. The new shim wraps fast_float::from_chars under
the _Py_dg_strtod calling convention.

fast_float's default `general` chars_format rejects leading '+' and
inf/nan literals. Enable:

  * allow_leading_plus — _Py_dg_strtod accepts "+3.14"; so does C
    strtod and test_float_with_comma's assertions depend on it.

Leave in place (not enabled):

  * skip_white_space — _Py_dg_strtod did not skip whitespace and the
    test_capi test_string_to_double asserts that " 0.1" raises
    ValueError. Higher-level callers (PyFloat_FromString) strip
    whitespace before the strtod call.

  * no_infnan stays ON so pystrtod.c's _Py_parse_inf_or_nan fallback
    keeps ownership of "inf"/"infinity"/"nan" literal parsing and
    continues to produce bit-identical results on those inputs.

Overflow and underflow map cleanly: fast_float returns
`std::errc::result_out_of_range` in both cases, populating `value`
with +/-inf on overflow and the nearest representable value on
underflow; the shim translates that to errno = ERANGE.

Python/dtoa.c is now completely unreachable from the rest of the
tree — commit 8 removes it. The full float-formatting + strtod +
C-API regression suite (1,501 tests across test_float, test_strtod,
test_format, test_capi) passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: eendebakpt

Objects/floatobject.c

@@ -940,7 +940,7 @@ 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 = _Py_fast_float_strtod(mybuf, NULL);
     _Py_SET_53BIT_PRECISION_END;
     if (errno == ERANGE && fabs(rounded) >= 1.)
         PyErr_SetString(PyExc_OverflowError,

Python/_fast_float/fast_float_strtod.cc

@@ -40,9 +40,13 @@ _Py_fast_float_strtod(const char *nptr, char **endptr)
     const char *last = nptr + std::strlen(nptr);
     double value = 0.0;
 
+    // fast_float's `general` default rejects leading '+' and "inf"/"nan".
+    // _Py_dg_strtod accepted leading '+' (C strtod does too) and left
+    // "inf"/"nan" to pystrtod.c's _Py_parse_inf_or_nan fallback — match both.
     auto result = fast_float::from_chars(
         nptr, last, value,
         fast_float::chars_format::general
+        | fast_float::chars_format::allow_leading_plus
         | fast_float::chars_format::no_infnan);
 
     if (result.ec == std::errc::invalid_argument) {

Python/pystrtod.c

@@ -1,7 +1,7 @@
 /* -*- Mode: C; c-file-style: "python" -*- */
 
 #include <Python.h>
-#include "pycore_dtoa.h"          // _Py_dg_strtod()
+#include "pycore_dtoa.h"          // _Py_fast_float_strtod(), _Py_fmt_dtoa()
 #include "pycore_pymath.h"        // _PY_SHORT_FLOAT_REPR
 
 #include <locale.h>               // localeconv()
@@ -101,7 +101,7 @@ _PyOS_ascii_strtod(const char *nptr, char **endptr)
     errno = 0;
 
     _Py_SET_53BIT_PRECISION_START;
-    result = _Py_dg_strtod(nptr, endptr);
+    result = _Py_fast_float_strtod(nptr, endptr);
     _Py_SET_53BIT_PRECISION_END;
 
     if (*endptr == nptr)