Add runnable example programs and update README example instructions

[blackopsrepl]

Motivation

• Provide small, self-contained example programs covering scalar, list, mixed, and classic CSP problems to demonstrate typical use-cases.
• Make README example instructions point to the crate-local examples and show concrete cargo run commands for each example variant.

Description

• Added four example binaries under crates/solverforge/examples/: scalar_graph_coloring.rs, list_tsp.rs, mixed_job_shop.rs, and nqueens.rs, each implementing a small, documented demo problem.
• Updated README.md examples section to reference crates/solverforge/examples/, added brief example type bullets (Scalar/List/Mixed) and replaced the single cargo run -p nqueens line with explicit cargo run -p solverforge --example ... invocations for each new example.

Testing

• Built the workspace examples with cargo build --examples, which completed successfully.
• Ran the test suite with cargo test, and all automated tests passed.

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Codex Task