Add a one-line Input component

[mrnasil]

Closes #26.

What and why

ratcn had twelve components and none of them took typed text. This adds Input: a one-line field whose string lives in app state and whose caret is a transient.

The caret overlays the insertion point as a blinking | and does not take a column, so moving or blinking it never shifts the letters. A left Down is ignored so the runtime can still move focus onto the field (the same press-to-focus path a checkbox uses); the following Click places the caret.

Word-kill is this field's own chord, but hosts disagree on the bytes. Matching only Backspace + Ctrl fails on Windows PowerShell and WSL, where ConPTY reports Ctrl+Backspace as Ctrl+H (0x08). The field therefore treats all of these as the same backward kill:

• Backspace with Control (kitty, browser)
• Ctrl+H / 0x08 (Windows ConPTY, xterm, WSL)
• Backspace with Alt (macOS Option, emacs-style)

Forward kill is Delete with Control (CSI 3;5~) or with Alt. Ctrl+W still bubbles.

Out of scope, on purpose: multi-line, password, selection, undo, max length.

Checks

• cargo fmt --all
• cargo test -p ratcn --all-features
• cargo check -p copy-fixture --examples
• cargo clippy -p ratcn --all-features --all-targets -- -D warnings
• cargo clippy --workspace --all-targets -- -D warnings
• cargo clippy --workspace --exclude cargo-ratcn --all-targets --target wasm32-unknown-unknown -- -D warnings

cargo test --workspace was not run to completion on this Windows host: cargo-ratcn has a pre-existing CRLF assertion in renders_the_expected_config, unrelated to this change.

If applicable

• Added a component? crates/copy-fixture/examples/input.rs added.
• Changed public API? Input, InputWidget, InputStyle, plus text_width::{grapheme_count, grapheme_byte, graphemes} so a copied module does not depend on unicode_segmentation. Docs page and changelog updated.
• Added a demo? demos/input/ with Trunk.toml.
• Added a dependency? No.
• Needs a newer Rust? No.