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/config.rs

@@ -0,0 +1,13 @@
+//! Input-related config enums (ZIG/src/input/config.zig).
+
+/// Determines the macOS option key behavior. See the config
+/// `macos-option-as-alt` for a lot more details.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
+#[repr(i32)]
+pub enum OptionAsAlt {
+    #[default]
+    False,
+    True,
+    Left,
+    Right,
+}