feat(input): port keyboard, keybinding and mouse input handling

Ports ZIG/src/input (except macOS keymaps) into ghostty-input:
- key, key_mods (incl. RemapSet), the physical keycode table
- key_encode: legacy, modifyOtherKeys and kitty keyboard protocol
  encoders, function key tables
- Binding: all keybinding actions with their user-visible docs (for
  +list-actions), triggers, sets with sequences, chains, leader keys and
  key tables, and formatting back to config strings
- command palette defaults, mouse, paste safety checks, kitty helpers,
  links, helpgen action docs

Deviations (docs/rewrite/notes/ghostty-input.md): Mods.super is named
super_; case folding uses char::to_lowercase plus Ghostty's special
cases instead of uucode tables; chain parents are trigger paths instead
of pointers; a Zig bug where chained leaves inside sequences corrupted the
shared prefix when formatting is fixed (covered by a test).

Integration left: building encode/paste Options from terminal modes,
and implementing the EntryFormatter trait in ghostty-config.

Test Plan:
- cargo test -p ghostty-input (220 tests; 8 macOS-only skipped)
- cargo clippy -p ghostty-input --all-targets -- -D warnings

crates/ghostty-input/src/lib.rs

@@ -1 +1,26 @@
-//! ghostty-input: see docs/rewrite/ARCHITECTURE.md for the module map.
+//! ghostty-input: keyboard/mouse input handling (ZIG/src/input/*).
+
+pub mod binding;
+pub mod command;
+pub mod config;
+pub mod function_keys;
+pub mod helpgen_actions;
+pub mod key;
+pub mod key_encode;
+pub mod key_mods;
+pub mod keyboard;
+pub mod keycodes;
+pub mod kitty;
+pub mod link;
+pub mod mouse;
+pub mod paste;
+
+pub use config::OptionAsAlt;
+pub use key::{Action, Key, KeyEvent, ctrl_or_super};
+pub use key_mods::{Mods, RemapSet as KeyRemapSet};
+pub use keyboard::Layout as KeyboardLayout;
+pub use link::Link;
+pub use mouse::{
+    Button as MouseButton, ButtonState as MouseButtonState, PressureStage as MousePressureStage,
+    ScrollMods,
+};