feat: add NK Landscape

[WojtAcht]

No description provided.

[Copilot]

Pull request overview

This PR introduces an NK Landscape discrete benchmark problem to lonkit, making it available as a built-in BitstringProblem for ILS-based discrete LON construction, and documents/exports it across the public API.

Changes:

• Added NKLandscape implementation (instance-seeded neighbor structure + contribution tables) with delta evaluation support.
• Exported NKLandscape from lonkit.discrete.problems, lonkit.discrete, and top-level lonkit.
• Added tests and documentation entries describing usage and parameters.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/discrete/test_problems.py	Adds unit tests covering determinism, neighbor models, delta evaluation, and local search properties for NKLandscape.
src/lonkit/discrete/problems/bitstring.py	Implements the NKLandscape problem.
src/lonkit/discrete/problems/init.py	Re-exports NKLandscape in the discrete problems package.
src/lonkit/discrete/init.py	Re-exports NKLandscape at the lonkit.discrete level.
src/lonkit/init.py	Re-exports NKLandscape at the top-level lonkit API.
README.md	Updates feature list to include NK Landscape as a built-in discrete problem.
docs/user-guide/discrete.md	Adds user guide section describing NKLandscape usage and parameters.
docs/index.md	Updates docs landing page feature summary to include NK Landscape.
docs/api/index.md	Adds NKLandscape to the API index list.
docs/api/discrete.md	Adds mkdocstrings entry for NKLandscape.
.gitignore	Ignores .gstack/.

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