feat: add contract CI pipeline

[Mrcee-arch]

• lint job: cargo fmt --check + cargo clippy -D warnings
• test job: cargo test --features testutils with llvm-cov coverage
  • posts coverage report as PR comment (creates/updates)
  • uploads lcov to Codecov with contract flag
• build-wasm job: compiles wasm32v1-none target, uploads unoptimized artifact
• optimize job: runs stellar contract optimize, uploads artifact (90-day retention on main, 7-day on PRs); enforces 64 KB size limit
• fuzz job: nightly schedule only, 10-minute cargo-fuzz run per target
• concurrency group cancels in-flight runs on new push

Summary

Changes

Testing

Checklist

• cargo fmt applied
• cargo clippy --features testutils -- -D warnings passes
• cargo test --features testutils passes
• New tests added for new behaviour
• CHANGELOG.md updated under [Unreleased]
• Docs updated if public API or behaviour changed

closes #212

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