docs: explain GixPython backend activation and requirements

Replace the historical Gitoxide introduction in `README.md` with an
explanation of `GixPython` as Gitoxide's Python bindings and the experimental
`GitPython` backend. Describe reduced subprocess overhead and reuse of the
native repository handle, with a link to the backend benchmarks.

Show installation from this unreleased source checkout and selection through
`GIT_PYTHON_BACKEND` before imports. Explain automatic selection, explicit
CLI/Gix choices, CPython 3.11+, Git 2.52+, and the Rust 1.89+ and system linker
needed for Linux and Windows source builds of `GixPython` 0.3.0. Its default
hashing, compression and HTTPS dependencies use pure Rust. Retain CLI fallback
and compatibility limitations, linking the backend ledger and downstream audit.

Resolve replay against the introducing commit's 0.3.0 upgrade by keeping the
new README structure while updating its package version and source-build
prerequisites. No repository implementation changes are part of this commit.

Validation: the README activation example returned the same HEAD as
`git rev-parse HEAD`; local links and `git diff --check` passed. The complete
fast profile passed: configured pre-commit hooks, `mypy --python-version=3.12`,
`basedpyright --warnings`, `uv sync --all-extras --all-groups --dry-run`, and
vendored-package CI lint. Full tests, builds, documentation builds and
benchmarks were outside the requested fast profile.

Author: codex

README.md

@@ -2,26 +2,64 @@
 [![Documentation Status](https://readthedocs.org/projects/gitpython/badge/?version=stable)](https://readthedocs.org/projects/gitpython/?badge=stable)
 [![Packaging status](https://repology.org/badge/tiny-repos/python:gitpython.svg)](https://repology.org/metapackage/python:gitpython/versions)
 
-## [Gitoxide](https://github.com/Byron/gitoxide): A peek into the future…
+## GixPython: Gitoxide for GitPython
 
-I started working on GitPython in 2009, back in the days when Python was 'my thing' and I had great plans with it.
-Of course, back in the days, I didn't really know what I was doing and this shows in many places. Somewhat similar to
-Python this happens to be 'good enough', but at the same time is deeply flawed and broken beyond repair.
+[GixPython](https://github.com/GitoxideLabs/GixPython) provides the **Python bindings
+for [Gitoxide](https://github.com/GitoxideLabs/gitoxide)**, an implementation of Git
+in [Rust](https://www.rust-lang.org). GitPython's experimental Gix backend uses these
+bindings to execute supported operations through Gitoxide while exposing GitPython's
+high-level repository, commit, tree and index interfaces.
 
-By now, GitPython is widely used and I am sure there is a good reason for that, it's something to be proud of and happy about.
-The community is maintaining the software and is keeping it relevant for which I am absolutely grateful. For the time to come I am happy to continue maintaining GitPython, remaining hopeful that one day it won't be needed anymore.
+Supported object reads, tree traversal, reference/index queries and commit history
+can run without launching Git subprocesses. GitPython also retains and reuses a Gix
+repository handle across operations, reducing repeated initialization work. These
+changes reduce subprocess overhead, particularly for repeated repository queries;
+the [backend benchmarks](test/performance/README.md) measure results and CLI launch
+counts for both backends.
 
-More than 15 years after my first meeting with 'git' I am still in excited about it, and am happy to finally have the tools and
-probably the skills to scratch that itch of mine: implement `git` in a way that makes tool creation a piece of cake for most.
+### Activate the Gix backend
 
-If you like the idea and want to learn more, please head over to [gitoxide](https://github.com/Byron/gitoxide), an
-implementation of 'git' in [Rust](https://www.rust-lang.org).
+This backend is currently unreleased. From this source checkout, install the
+optional `gix` extra in a virtual environment:
 
-The experimental `GitPython[gix]` extra uses GixPython 0.3.0 from PyPI for
-supported operations, with Git CLI fallback. Set `GIT_PYTHON_BACKEND=cli` or
-`gix` to select the backend explicitly. Tests use `gix.testtools` for scripted
-fixtures with either backend and require Python 3.11+. See the
-[installation, test commands, and conversion ledger](doc/gix-backend.md).
+```sh
+python -m pip install ".[gix]"
+```
+
+The extra installs **GixPython 0.3.0** from PyPI; its Python import name is `gix`.
+Set `GIT_PYTHON_BACKEND=gix` **before importing GitPython**, either in your
+environment or in Python:
+
+```python
+import os
+
+os.environ["GIT_PYTHON_BACKEND"] = "gix"
+from git import Repo
+
+with Repo(".") as repo:
+    print(repo.head.commit.hexsha)
+```
+
+With `GIT_PYTHON_BACKEND` unset, GitPython automatically selects Gix when `gix`
+can be imported, otherwise CLI. Set it to `cli` to force Git subprocesses instead.
+Explicit `gix` selection fails if GixPython cannot be imported.
+
+### Prerequisites and coverage
+
+- **CPython 3.11 or newer** for GixPython. GitPython itself still supports Python
+  3.8 or newer; on Python 3.8–3.10 the extra omits GixPython and uses CLI.
+- **Git 2.52 or newer**, available in `PATH` or selected with
+  `GIT_PYTHON_GIT_EXECUTABLE`, remains required for CLI fallbacks.
+- GixPython 0.3.0 publishes **macOS wheels**. On **Linux and Windows**, installation
+  builds its released source distribution and requires **Rust 1.89 or newer** and
+  a **system linker**. Default hashing, compression and HTTPS dependencies use
+  pure Rust and do not require a C/C++ compiler.
+
+Unsupported operations and options retain the Git CLI path, including clone,
+fetch and push. Direct `repo.git.<command>()` calls also retain command-line
+behavior. Coverage and compatibility gaps are documented in the
+[backend ledger and installation guide](doc/gix-backend.md); the
+[downstream audit](test/downstream/audit.md) starts with known API incompatibilities.
 
 ## GitPython