Publish to crates.io

[thomasnemer]

Set up publishing to crates.io so users can install syld via cargo install syld.

Tasks:

• Verify metadata in Cargo.toml (description, license, repository, etc.)
• Add a publish step to the release workflow or document the manual process
• cargo publish --dry-run to validate before first publish

[thomasnemer]

Deferring until v1.0.0 — the CLI surface and feature set are still evolving. Publishing to crates.io before stabilization would create unnecessary churn for consumers.