gh-NNNN: Vendor {fmt} 12.1.0 float path into Python/_fmt/

Drops the four files needed to replace _Py_dg_dtoa's float-to-string
role: format.h, format-inl.h, base.h, and the upstream LICENSE. No
integration yet — later commits add the C ABI shim (fmt_dtoa.cc), the
build rules, and the pystrtod.c/floatobject.c call-site changes.

Files are taken unmodified from upstream fmt v12.1.0 (commit 407c905),
MIT-licensed. See Python/_fmt/README.vendor for the trimming rationale
and a refresh procedure.

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

Author: eendebakpt

Python/_fmt/LICENSE

@@ -0,0 +1,27 @@
+Copyright (c) 2012 - present, Victor Zverovich and {fmt} contributors
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+--- Optional exception to the license ---
+
+As an exception, if, as a result of your compiling your source code, portions
+of this Software are embedded into a machine-executable object form of such
+source code, you may redistribute such embedded portions in such object form
+without including the above copyright and permission notices.

Python/_fmt/README.vendor

@@ -0,0 +1,43 @@
+Vendored subset of the {fmt} library, used by Python/pystrtod.c for
+float-to-string conversion (replaces the David Gay dtoa machinery that
+lived in Python/dtoa.c).
+
+Upstream:    https://github.com/fmtlib/fmt
+Version:     12.1.0 (tag v12.1.0, commit 407c905)
+License:     MIT (see LICENSE in this directory)
+
+Files included:
+
+    format.h        — core float formatting path, Dragon4 bignum, Dragonbox
+                      shortest entry point declaration
+    format-inl.h    — out-of-line Dragonbox tables and helpers; #include'd
+                      exactly once from fmt_dtoa.cc
+    base.h          — buffer, basic_fp, and utility types that format.h
+                      depends on
+    LICENSE         — upstream MIT license text
+
+Deliberately excluded: chrono.h, color.h, compile.h, core.h, os.h,
+ostream.h, printf.h, ranges.h, std.h, xchar.h, args.h. None of these are
+reachable from the float path; keeping them out of the tree avoids
+dragging locale/iostream/chrono dependencies into libpython.
+
+The actual bridge between fmt and CPython is Python/_fmt/fmt_dtoa.cc,
+which wraps fmt::detail::format_float and fmt::detail::dragonbox::to_decimal
+behind an extern "C" _Py_fmt_dtoa entry point matching the old
+_Py_dg_dtoa calling convention.
+
+Build configuration:
+
+    $(CXX) -std=c++17 -fno-exceptions -fno-rtti
+
+fmt auto-detects -fno-exceptions via __cpp_exceptions and replaces its
+throw sites with assertion failures. No RTTI is required on the float
+path.
+
+Updating:
+
+    git clone --branch vX.Y.Z --depth 1 https://github.com/fmtlib/fmt.git
+    cp fmt/include/fmt/{format.h,format-inl.h,base.h} Python/_fmt/
+    cp fmt/LICENSE Python/_fmt/
+
+Then update this file's version and commit hash.