fix: tombstone build needs setuptools installed explicitly

[Jammy2211]

Follow-up to #240, part of #238.

The TestPyPI rehearsal (run 32309423898) failed — which is what a rehearsal is for. It failed at build, before the upload step, so nothing was published and no version was burned (test.pypi.org has no autolens 2026.7.29.1.post1).

Cause

The sdist builds with --no-isolation — deliberate, so the build cannot silently reach the network for a backend. Python 3.12 dropped setuptools from the default environment, and the workflow installed only build and twine. The result:

ERROR Backend 'setuptools.build_meta:__legacy__' is not available.
pyproject_hooks._impl.BackendUnavailable: Cannot import 'setuptools.build_meta'

That message names neither the cause nor the fix. It did not reproduce locally because this machine's 3.12 happens to carry setuptools — a bare 3.12 venv does not, which is exactly the runner's condition.

Fix

• publish_tombstone.yml installs setuptools explicitly.
• build_sdist() checks for the backend first and raises a message saying what to install, so the next environment without it gets an answer rather than a stack trace.
• test_missing_setuptools_is_reported_before_the_build pins the precondition.

Verification

Reproduced the runner's condition in a bare 3.12 venv (find_spec("setuptools") is None):

• before installing setuptools → the new check fires with the actionable message;
• after installing it → all five sdists build, each declaring Requires-Python <3.12, and twine check PASSED on every one.

That last step matters beyond this bug: it is the first confirmation that the tombstone metadata is acceptable to a package index at all.

Full suite: 330 passed.

API Changes

None. Build tooling only; no PyAuto* library API is touched.

🤖 Generated with Claude Code