gh-NNNN: Route float→string through _Py_fmt_dtoa

Swap the _Py_dg_dtoa / _Py_dg_freedtoa call pair at its two in-tree call
sites (pystrtod.c's format_float_short and floatobject.c's
double_round) for _Py_fmt_dtoa / _Py_fmt_dtoa_free. Covers all three
CPython dtoa modes bit-exactly:

  * mode 0 (shortest / repr): fmt::detail::dragonbox::to_decimal
  * mode 2 (N sig digits):    fmt::detail::format_float, specs=exp
  * mode 3 (N frac digits):   fmt::detail::format_float, specs=fixed

fmt's format_float with specs=exp takes `precision` as total-digits-to-
emit (not "digits after the leading one"), so for mode 2 we pass N
directly. For specs=fixed fmt's adjust_precision adds the decade and
handles negative precision natively, which is what mode 3 with negative
ndigits wants.

Python/dtoa.c is still in the build for _Py_dg_strtod (commit 7 handles
that swap) but _Py_dg_dtoa is now unreferenced.

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

Author: eendebakpt

Objects/floatobject.c

@@ -900,7 +900,8 @@ float___ceil___impl(PyObject *self)
 
 #if _PY_SHORT_FLOAT_REPR == 1
 /* version of double_round that uses the correctly-rounded string<->double
-   conversions from Python/dtoa.c */
+   conversions from the fmt / fast_float shims (Python/_fmt/,
+   Python/_fast_float/). */
 
 static PyObject *
 double_round(double x, int ndigits) {
@@ -914,7 +915,7 @@ double_round(double x, int ndigits) {
 
     /* round to a decimal string */
     _Py_SET_53BIT_PRECISION_START;
-    buf = _Py_dg_dtoa(x, 3, ndigits, &decpt, &sign, &buf_end);
+    buf = _Py_fmt_dtoa(x, 3, ndigits, &decpt, &sign, &buf_end);
     _Py_SET_53BIT_PRECISION_END;
     if (buf == NULL) {
         PyErr_NoMemory();
@@ -951,7 +952,7 @@ double_round(double x, int ndigits) {
     if (mybuf != shortbuf)
         PyMem_Free(mybuf);
   exit:
-    _Py_dg_freedtoa(buf);
+    _Py_fmt_dtoa_free(buf);
     return result;
 }
 

Python/pystrtod.c

@@ -980,11 +980,12 @@ format_float_short(double d, char format_code,
     Py_ssize_t decpt, digits_len, vdigits_start, vdigits_end;
     _Py_SET_53BIT_PRECISION_HEADER;
 
-    /* _Py_dg_dtoa returns a digit string (no decimal point or exponent).
-       Must be matched by a call to _Py_dg_freedtoa. */
+    /* fmt-backed drop-in for _Py_dg_dtoa. Returns a digit string (no
+       decimal point or exponent). Must be matched by a call to
+       _Py_fmt_dtoa_free. Covers all three modes bit-exactly. */
     _Py_SET_53BIT_PRECISION_START;
-    digits = _Py_dg_dtoa(d, mode, precision, &decpt_as_int, &sign,
-                         &digits_end);
+    digits = _Py_fmt_dtoa(d, mode, precision, &decpt_as_int, &sign,
+                          &digits_end);
     _Py_SET_53BIT_PRECISION_END;
 
     decpt = (Py_ssize_t)decpt_as_int;
@@ -1212,7 +1213,7 @@ format_float_short(double d, char format_code,
         assert(p-buf < bufsize);
     }
     if (digits)
-        _Py_dg_freedtoa(digits);
+        _Py_fmt_dtoa_free(digits);
 
     return buf;
 }