Preserve Kitty keyboard event metadata

[robinvd]

Summary

• preserve canonical, shifted, and base-layout keys from Kitty CSI-u events
• expose associated text without clearing Shift or replacing the canonical key
• validate full modifier/event fields, including encoded mask 256
• parse all keyboard enhancement report flags
• initialize enhancement metadata on Unix and Windows event paths

Motivation

Terminal multiplexers need both physical key identity and generated text to re-encode input faithfully. The current parser replaces the canonical key with the shifted key, clears Shift, and drops base-layout and associated-text fields, which makes combinations such as Shift+1 impossible to forward correctly.

Validation

• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo check --target x86_64-pc-windows-gnu --features windows-legacy

KeyEvent is no longer Copy because associated text is variable-length; this is an intentional API change.

[the-mikedavis]

Dropping Copy is unfortunate but I don't see a way around it, maybe other than limiting the associated text to a fixed-size buffer and truncating larger inputs. But that doesn't really seem reasonable.

Is the idea to use this for something like Zellij?