Release-readiness cleanup for license recognition and package layout

[Copilot]

This PR tightens release-readiness in two places that were still rough on main: GitHub license detection and package structure. It normalizes the repository to a canonical Apache-2.0 license and moves the core implementation into a proper package without changing the public import path.

• License normalization

  • Replaced the repository LICENSE contents with the canonical Apache License 2.0 text.
  • Kept packaging metadata aligned with Apache-2.0 so repository and distribution metadata describe the same license.

• Package layout refactor

  • Moved the core implementation from the repo root into a dedicated package:
    • jump_diffusion_engine/engine.py
    • jump_diffusion_engine/__init__.py
  • Preserved the public API surface by re-exporting JumpDiffusionEngine from the package root.

• Packaging metadata

  • Updated pyproject.toml from single-module packaging to package discovery.
  • Kept versioning unchanged at 0.1.0.

• Release-facing docs and metadata

  • Updated README and changelog references that still described the old root-module layout.
  • Kept install/import guidance consistent with GitHub release-based installation and the package refactor.

• Tests

  • Removed the test-side repo-root import workaround.
  • Added a focused assertion that the public import resolves through the new package layout.

Example import remains unchanged:

from jump_diffusion_engine import JumpDiffusionEngine