Install hatchling in release CI so --no-isolation wheel build works

build_wheels.sh runs `python -m build --no-isolation`, which requires the
build backend (hatchling) to already be importable. The release workflow only
installed `build wheel`, so the wheel build failed with 'Backend
hatchling.build is not available'. Add hatchling to the install step and to the
documented build requirements.

Author: rahlk

.github/workflows/release.yml

@@ -80,7 +80,9 @@ jobs:
           python-version: "3.11"
 
       - name: Install Python build tooling
-        run: python -m pip install --upgrade build wheel
+        # hatchling is the build backend; build_wheels.sh runs `python -m build
+        # --no-isolation`, so it must be present in this env (no auto-install).
+        run: python -m pip install --upgrade build wheel hatchling
 
       - name: Build platform wheels (cross-compiles every target via Bun)
         working-directory: packaging/python

packaging/python/README.md

@@ -19,7 +19,7 @@ sdist: the binary cannot be built without Bun.
 ## Building & publishing
 
 ```bash
-python -m pip install build wheel twine
+python -m pip install build wheel hatchling twine
 ./build_wheels.sh            # cross-compiles every target via Bun, emits ./dist/*.whl
 twine upload dist/*.whl
 ```

packaging/python/build_wheels.sh

@@ -9,7 +9,8 @@
 #
 # Requirements on the build host:
 #   - bun            (https://bun.sh)  -- cross-compiles all targets from one host
-#   - python -m pip install build wheel twine
+#   - python -m pip install build wheel hatchling twine
+#     (hatchling is the build backend; --no-isolation means it must be installed)
 #
 # Usage:
 #   ./build_wheels.sh           # build all targets into ./dist