fix(examples): add cougr-core dependency to checkers

[KevinMB0220]

Description

This PR fixes the dependency issue in the checkers example as outlined in #134.

Previously, examples/checkers/src/lib.rs used cougr_core, but examples/checkers/Cargo.toml did not explicitly declare it. This prevented the example from serving as a reliable standalone reference that could be built in isolation.

Changes:

• Explicitly added cougr-core = "1.0.0" (the published crate version) to the [dependencies] section in examples/checkers/Cargo.toml.

Validation Performed

• ✅ Verified that cargo test runs successfully from the examples/checkers directory.
• ✅ Verified that stellar contract build completes successfully for the checkers example.
• ✅ Ensured no unrelated examples were modified.

Related Issues

• Closes Fix checkers Cougr dependency #134

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[salazarsebas]

LGTM