test: add ExperimentalFeatureWarning unit + integration tests
Add Python unit tests verifying the __getattr__ compat shim works:
import succeeds, deprecation prints to stderr once, is FutureWarning
subclass, works with warnings.filterwarnings, unknown attrs raise.
Add txtar integration test proving a predictor importing
ExperimentalFeatureWarning builds and runs end-to-end.
Also fix coglet wheel build/test workflow:
- noxfile: fail loudly when dist/ has wheels but none match platform
(was silently falling back to PyPI or installing wrong-platform wheel)
- noxfile: always use editable install for SDK so tests run against
working tree
- mise.toml: remove sources/outputs caching from build:coglet:wheel
(the broad glob falsely matched cross-compiled wheels, causing skips)
- mise.toml: test:python depends on build:coglet:wheel so dist/ always
has a native wheel
