Prepare stack-theme for crates.io

[konojunya]

Summary

• declare crates.io as the only publication registry and add discovery metadata for stack-theme
• document the versioned Cargo dependency and Rust 1.85 support floor
• keep the published package constrained to generated catalog code, schemas, repository-authored assets, docs, and the Apache-2.0 license
• upgrade the existing package gate to Cargo's crates.io publish dry run

Testing

• npm run validate
• npm run generate:check
• npm test
• npm run typecheck
• npm pack --dry-run --workspace @stack-sh/theme
• cargo fmt --check
• cargo test --workspace --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --workspace --no-deps --locked
• cargo publish --dry-run -p stack-theme --locked --allow-dirty
• cargo +1.85.0 test --workspace --locked
• cargo +1.85.0 clippy --workspace --all-targets --locked -- -D warnings