Accept typed callables as scalar objectives

[isty2e]

Summary

Problem now accepts a typed scalar callable through objective= and normalizes it at construction into the existing Objective and evaluation-protocol path.

This keeps callable convenience at the public boundary without adding another execution model. Direction handling, failure recording, evaluation accounting, diagnostics, and the direct scalar sequential fast path continue to use the established contracts. Callable-backed problems also retain stable equality and hashing when they wrap the same callable object.

The README, quickstart, evaluator guide, and changelog now show the shorter callable form and explain the portability boundary. Importable module-level functions remain the conservative choice when evaluation may cross process or MPI boundaries.

Validation

• uv run ruff check --extend-select I,UP pyproject.toml src tests .github/scripts
• uv run ruff format --check pyproject.toml src tests .github/scripts
• uv run --extra test pre-commit run --all-files
• uv run basedpyright
• uv run pytest (1702 passed)
• uv run mkdocs build --strict

Closes #96