feat: add CI workflow and crates.io publishing metadata

[montfort]

Summary

• Add cross-platform CI workflow (ubuntu, macos, windows) with cargo fmt --check, cargo clippy, and cargo test
• Add crates.io metadata to Cargo.toml: repository, homepage, keywords, categories, readme, exclude
• Fix pre-existing rustfmt issues in test files
• Validated with cargo publish --dry-run

Test plan

• cargo publish --dry-run passes
• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• cargo test — 29 tests pass
• CI workflow runs green on GitHub

🤖 Generated with Claude Code

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