API documentation and README with per-platform examples

[enzoftware]

Description

Write comprehensive documentation: Rust API docs (cargo doc), and a README with usage examples for every platform target.

Tasks

• Add /// doc comments to all public types and functions in silvestre-core
• Add /// doc comments to FFI functions
• Write README.md with:
  • Project overview and architecture diagram
  • Quick start for each platform (Rust, CLI, WASM, Flutter)
  • Code examples for common operations
  • Build instructions
  • Contributing guidelines
• cargo doc --no-deps --workspace generates clean documentation
• Add inline examples in doc comments (/// # Examples) that compile with cargo test --doc

Acceptance Criteria

• cargo doc produces documentation with no warnings
• Every public item has a doc comment
• README covers all 5 platform targets with working examples
• Doc tests compile and pass