chore: prepare for 3.0.0 (final) (#5746)

* Update docs/changelog.md and change version to v3.0.0 (final)

* [skip ci] Add `|SPEC 4 — Using and Creating Nightly Wheels|` badge in main README.rst

Author: rwgk

README.rst

@@ -3,7 +3,9 @@
 
 **pybind11 (v3)  — Seamless interoperability between C++ and Python**
 
-|Latest Documentation Status| |Stable Documentation Status| |Gitter chat| |GitHub Discussions| |CI| |Build status|
+|Latest Documentation Status| |Stable Documentation Status| |Gitter chat| |GitHub Discussions|
+
+|CI| |Build status| |SPEC 4 — Using and Creating Nightly Wheels|
 
 |Repology| |PyPI package| |Conda-forge| |Python Versions|
 
@@ -210,3 +212,5 @@ to the terms and conditions of this license.
    :target: https://pypi.org/project/pybind11/
 .. |GitHub Discussions| image:: https://img.shields.io/static/v1?label=Discussions&message=Ask&color=blue&logo=github
    :target: https://github.com/pybind/pybind11/discussions
+.. |SPEC 4 — Using and Creating Nightly Wheels| image:: https://img.shields.io/badge/SPEC-4-green?labelColor=%23004811&color=%235CA038
+   :target: https://scientific-python.org/specs/spec-0004/

docs/changelog.md

@@ -12,13 +12,7 @@ versioning](http://semver.org) policy.
 Changes will be added here periodically from the "Suggested changelog
 entry" block in pull request descriptions.
 
-## 3.0.0 (RC 3) (June 4, 2025)
-
-Since this is a large release, we are providing a release candidate to give
-projects time to test! We also now provide
-[SPEC 4](https://scientific-python.org/specs/spec-0004/) nightly wheels. We
-are hoping to split up `std.h`; that work is approved to be added during the
-RC phase if it's ready in time. We expect the RC phase to last around a week.
+## 3.0.0 (final) (July 10, 2025)
 
 Pybind11 3.0 includes an ABI bump, the first required bump in many years
 on Unix (Windows has had required bumps more often). This release contains
@@ -129,8 +123,10 @@ New Features:
 
 - `pybind11/conduit/pybind11_platform_abi_id.h` was factored out, to
   maximize reusability of `PYBIND11_PLATFORM_ABI_ID` (for other
-  Python/C++ binding systems).
-  [#5375](https://github.com/pybind/pybind11/pull/5375)
+  Python/C++ binding systems). Separately, a note was added to explain
+  that the conduit feature only covers from-Python-to-C++ conversions.
+  [#5375](https://github.com/pybind/pybind11/pull/5375) \|
+  [#5740](https://github.com/pybind/pybind11/pull/5740)
 
 - Added support for finding pybind11 using pkgconf distributed on pypi.
   [#5552](https://github.com/pybind/pybind11/pull/5552)

include/pybind11/detail/common.h

@@ -24,10 +24,10 @@
 // - The release level is set to "alpha" for development versions.
 //   Use 0xA0 (LEVEL=0xA, SERIAL=0) for development versions.
 // - For stable releases, set the serial to 0.
-#define PYBIND11_VERSION_RELEASE_LEVEL PY_RELEASE_LEVEL_GAMMA
-#define PYBIND11_VERSION_RELEASE_SERIAL 4
+#define PYBIND11_VERSION_RELEASE_LEVEL PY_RELEASE_LEVEL_FINAL
+#define PYBIND11_VERSION_RELEASE_SERIAL 0
 // String version of (micro, release level, release serial), e.g.: 0a0, 0b1, 0rc1, 0
-#define PYBIND11_VERSION_PATCH 0rc4
+#define PYBIND11_VERSION_PATCH 0
 /* -- end version constants -- */
 
 #if !defined(Py_PACK_FULL_VERSION)