Ensure example stacks lock in CI

[ncoghlan]

There is currently some test coverage that is only obtained by running misc/lock-example-stacks.sh --build.

More specifically, that coverage is gained by locking & building the pytorch example stack (as that covers using uv's source configuration with actual mutually incompatible indexes).

It has been omitted from CI due to concerns that it may take too long to run with a cold uv cache.

This issue covers checking whether those concerns are valid, and then either obtaining that coverage another way (if the torch based test is indeed too slow), or else adding the torch example to the test suite as a new test case.

[ncoghlan]

One potential way to get a cheaper test case would be to add a variant of the local wheels test case that creates a derived spec that defines a couple of "flat" local indexes (one for the publisher package, one for the consumer package) instead of relying on the local wheel directory CLI option.

[ncoghlan]

Given how slow some of the existing slow tests are, this might not be too bad by comparison. Locking all three current example stacks locally with a cold uv cache only takes a couple of minutes (the example stack locking script forces lock resets and rebuilding the environments):

real    1m51.240s
user    0m38.624s
sys     0m36.821s

For comparison, a timed slow test run on this machine with a cold uv cache gives:

130.43s call     tests/test_sample_project.py::TestBuildEnvironment::test_build_is_reproducible
43.01s call     tests/test_minimal_project.py::TestMinimalBuild::test_locking_and_publishing
7.60s call     tests/test_local_wheels_project.py::TestBuildEnvironment::test_locking_and_publishing
6.80s call     tests/test_env_locks.py::test_build_env_layer_locks
6.57s setup    tests/test_local_wheels_project.py::TestBuildEnvironment::test_create_environments
6.40s call     tests/test_sample_project.py::TestBuildEnvironment::test_create_environments
4.70s call     tests/test_local_wheels_project.py::TestBuildEnvironment::test_create_environments

Still, even though it's comparable to the existing runtime of the test_sample_project build reproducibility checks, those checks are themselves roughly a third of the existing runtime for the full test suite.

[ncoghlan]

To reduce wall clock time, adding pytest-xdist as a CI dependency and then running with -n auto --dist worksteal should make it possible to have the slow tests run in the time it takes test_sample_project to run, as long as the system has at least two physical cores.

[ncoghlan]

Most of the examples demonstrate something interesting that the main test suite may not fully exercise, so the issue title has been expanded to reflect that.

[ncoghlan]

As part of resolving #333, an intermediate improvement for this situation is being added: a test case that checks the example layer locks are up to date, and performs various validations on their contents.