feat(textarea): add native cursor render mode (#26)

* feat(textarea): add native cursor render mode

- Add CursorRenderMode::{Cell, Hidden} and TextArea accessors for render mode plus rendered_cursor_position().
- Store the terminal-relative cursor position from a shared render plan that accounts for blocks, line numbers, scroll, wrapping, tabs, and wide Unicode.
- Let hidden mode suppress only the textarea-owned cursor cell while preserving default Cell rendering and set_cursor_style behavior.
- Document native cursor integration and add focused tests for default rendering, hidden rendering, placeholders, and cursor-position edge cases.

* chore: prepare 0.12.1 release

Cargo.toml

@@ -3,7 +3,7 @@ name = "tui-textarea-2"
 homepage = "https://github.com/srothgan/tui-textarea#readme"
 repository = "https://github.com/srothgan/tui-textarea"
 documentation = "https://docs.rs/tui-textarea-2/latest/tui_textarea/"
-version = "0.12.0"
+version = "0.12.1"
 edition = "2024"
 rust-version = "1.88.0" # for Ratatui 0.30 split crates
 authors = ["Simon Peter Rothgang <simonrothgang@icloud.com>", "rhysd <lin90162@yahoo.co.jp>"]