[dbrattli]

Invalid downcasts previously relied on assertions that disappeared under python -O, and try_downcast(list[int], ["wrong"]) could succeed by discarding generic arguments. Use explicit runtime checks: incompatible upcast and downcast values raise TypeError, while try_downcast returns None for a value mismatch. Unsupported parameterized targets now raise TypeError rather than claiming unchecked element types.

Keep the uncurried upcast(Base, value) API and infer its return type from the target alone. Document that this form checks compatibility at runtime, with a TODO to adopt upcast(Base)(value) for static enforcement in the next major release. Successful casts preserve object identity.

Compatibility changes: failed downcasts raise TypeError instead of AssertionError; invalid upcasts now fail; parameterized targets, including list[Any], are rejected. Add identity, inference, negative typing, and optimized-interpreter regressions, and update the casting documentation.

Verification: .venv/bin/pytest -q and PATH="$PWD/.venv/bin:$PATH" .venv/bin/pre-commit run --all-files.