Add .editorconfig matching existing Rust and Markdown formatting #73

Create .editorconfig at the repo root. Existing files (src/lib.rs, src/main.rs, tests/cli.rs, README.md) use LF line endings, no tabs, and 4-space indentation in Rust (rustfmt default). Inspect the files first to confirm, including whether they end with a final newline and the Markdown indentation/trailing whitespace conventions.

Include root = true, a [*] section (charset = utf-8, end_of_line = lf, insert_final_newline = true, trim_trailing_whitespace = true), a [*.rs] section (indent_style = space, indent_size = 4), and a [*.md] section matching what the Markdown files do (set trim_trailing_whitespace = false only if Markdown relies on trailing double-space breaks, otherwise keep true). Optionally [*.toml] with 2 or 4 spaces to match Cargo.toml. Do not reformat any existing file.

Definition of done

• test -f .editorconfig passes.
• grep -q 'root = true' .editorconfig passes.
• cargo test passes.