ci(gix): add Ubuntu and macOS jobs with Cargo caching

The full-suite matrix only installed `.[test,gix]` on Windows, so Ubuntu
and macOS exercised the CLI backend even where `GixPython` was available.
Add one Python 3.12 Gix job on each platform, covering all three runner
OSes while preserving the 28 existing CLI jobs and their check names.

Use the official PyPI release: macOS can install wheels, while Linux and
Windows build the source distribution with stable Rust. Keep
`CARGO_TARGET_DIR` outside `pip`'s temporary source directory and cache it
alongside Cargo registry and Git dependencies. Cache keys include OS,
architecture, Python, Rust and `gix-requirements.txt`, allowing dependency
reuse across releases on the same platform and toolchain. Retain the
existing `pip` wheel cache and document the job names and build requirements.

Validation:

- `actionlint` 1.7.12 and the repository's pinned `pre-commit` checks passed.
  Matrix expansion verifies 31 jobs and unchanged CLI combinations/names.
- Built the SHA-256-verified `GixPython` 0.1.0 source distribution using the
  existing Python 3.12.14 and Rust 1.99.0. The first build, including test
  environment setup, took 117.51 seconds. A rebuild from a different source
  path reused all 228 Cargo crates and completed in 1.57 seconds.
- Ran the workflow's installation/backend assertion with that source-built
  wheel and verified the installed extension matches it byte-for-byte.
- Focused repository/backend tests passed with Gix first: 16 tests and
  14 subtests. CLI then passed 12 tests and 14 subtests, with the Gix-only
  module skipped. These are local checks; the new CI jobs have not run yet.

Author: codex

.github/workflows/pythonpackage.yml

@@ -34,6 +34,16 @@ jobs:
           experimental: true
         - python-version: "3.15t"
           experimental: true
+        - os-type: ubuntu
+          python-version: "3.12"
+          backend: gix
+          build-docs: false
+          experimental: false
+        - os-type: macos
+          python-version: "3.12"
+          backend: gix
+          build-docs: false
+          experimental: false
         - os-type: windows
           python-version: "3.12"
           backend: gix
@@ -65,13 +75,27 @@ jobs:
           test-requirements.txt
           gix-requirements.txt
 
-    - name: Prepare Rust for GixPython's Windows source build
+    - name: Prepare Rust for GixPython source builds
       if: matrix.backend == 'gix'
+      id: rust
       run: |
         # GixPython 0.1.0 publishes macOS wheels only and needs Rust >= 1.89.
         rustup toolchain install stable --profile minimal
         rustup default stable
         rustc --version
+        echo "version=$(rustc --version)" >> "$GITHUB_OUTPUT"
+
+    - name: Cache GixPython Cargo dependencies and build output
+      if: matrix.backend == 'gix'
+      uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+      with:
+        path: |
+          ~/.cargo/registry
+          ~/.cargo/git
+          ${{ runner.temp }}/gix-target
+        key: gix-${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-${{ steps.rust.outputs.version }}-${{ hashFiles('gix-requirements.txt') }}
+        restore-keys: |
+          gix-${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-${{ steps.rust.outputs.version }}-
 
     - name: Set up WSL (Windows)
       if: matrix.os-type == 'windows'
@@ -115,6 +139,8 @@ jobs:
       env:
         EXTRAS: ${{ matrix.backend == 'gix' && 'test,gix' || 'test' }}
         EXPECTED_BACKEND: ${{ matrix.backend }}
+        # Keep Cargo output when pip removes its temporary source directory.
+        CARGO_TARGET_DIR: ${{ runner.temp }}/gix-target
 
     - name: Show POSIX file ownership
       # Linux and macOS only. On Windows, Git Bash's `ls -ld` reports a

doc/gix-backend.md

@@ -18,8 +18,9 @@ uv pip install --python .tox/gix/bin/python --editable '.[test,gix]'
 
 GixPython 0.1.0 publishes macOS wheels. On Windows and Linux, installation
 builds the released source distribution and requires Rust 1.89 or newer and
-a platform C/C++ toolchain. Windows CI uses the runner's MSVC tools and a
-stable Rust toolchain, with pip's built-wheel cache enabled.
+a platform C/C++ toolchain. The full-suite Gix CI jobs prepare a stable Rust
+toolchain and cache pip's built wheels, Cargo dependencies and compilation
+output.
 
 Python extras add dependencies; they do not leave a runtime feature bit.
 GitPython selects this backend when `gix` can be imported. Installing GixPython
@@ -848,16 +849,25 @@ Add each conversion with a no-subprocess check and a Git parity check where
 practical, in its own commit. Update this ledger when a limitation changes;
 the runtime report's reason should point to the corresponding entry.
 
-## Windows validation
+## CI coverage
 
-The [Python package workflow](../.github/workflows/pythonpackage.yml) adds a
-Windows/Python 3.12 Gix job while preserving all 28 CLI combinations and their
-existing check names. It installs `.[test,gix]`, verifies the selected backend
-and runs the full suite
-with the same coverage and pytest options. Every job retains JUnit results
+The [Python package workflow](../.github/workflows/pythonpackage.yml) adds one
+Python 3.12 Gix job each on Ubuntu, macOS and Windows, preserving all 28 CLI
+combinations and their existing check names. It installs `.[test,gix]`, verifies
+the selected backend and runs the full suite with the same coverage and pytest
+options. Every job retains JUnit results
 and `--backend-report` operation counts under its `tests-OS-PYTHON-BACKEND`
-artifact, and prints the 30 slowest test durations. The extra Gix job does not
-duplicate the documentation build.
+artifact, and prints the 30 slowest test durations. The extra Gix jobs do not
+duplicate the documentation build. Their check names include `gix`, such as
+`test (ubuntu, 3.12, gix)`.
+
+On platforms without a matching wheel, pip builds the official source release.
+`CARGO_TARGET_DIR` keeps compilation output outside pip's temporary source
+directory. The Cargo cache distinguishes OS, architecture, Python and Rust
+versions and `gix-requirements.txt`; a release change can reuse dependencies
+from the same platform and toolchain.
+
+## Windows validation
 
 Local Windows checks use CPython 3.12.13, Git 2.55.0.windows.3 and GixPython
 0.1.0 built from the released source distribution. The installed extension