[SizzleUnrlsd]

Closes #140. This is the engine hook enterprise-validators (CoreTrace-Entreprise/coretrace-python-analyzer-plugins#25) needs, so a project can declare its validators in a file at its root.

Change

• Plugin.project_models(root) -> Sequence[Model]. It defaults to (), and returns models read from the project under analysis. plugin_models(plugins, root) registers them next to the plugin's own models, under the plugin's name, so a conflict still names both sides.
• Where it runs. It is called once per directory check, before any module is analysed. With --jobs, each worker calls it with the batch root, so every process sees the same models. A single-file check reads no project.
• Cache. The models join the model table, which is already in the configuration key. Editing the declaration therefore invalidates cached results; a test covers it.
• Errors. A model declared wrongly raises ModelError. The command line now lists ModelError among the errors it reports (exit status 2), so a bad declaration gives a message instead of a traceback. This also covers the existing case of two plugins declaring conflicting models.

Verification

• pytest: 1081 passed. ruff check src tests and mypy are clean.
• pytest -m regression under Python 3.12: 19 passed, no snapshot changed. No bundled plugin reads the project.
• Tests. Before the change, 4 of the 5 new tests failed for the right reasons: the declared validator was ignored (hotspot), in-process, in workers and through the cache, and the command line exited with 1 instead of reporting the error. The single-file case was already green, and stays so.
• Mutation checks. Each of these changes fails at least one test: the project path skipping the hook, workers skipping it, and the command line letting ModelError through.