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/keyboard.rs

@@ -0,0 +1,56 @@
+//! Keyboard layouts (ZIG/src/input/keyboard.zig).
+
+use crate::config::OptionAsAlt;
+
+/// Keyboard layouts.
+///
+/// These aren't heavily used in Ghostty and having a fully comprehensive
+/// list is not important. We only need to distinguish between a few
+/// different layouts for some nice-to-have features, such as setting a default
+/// value for "macos-option-as-alt".
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Layout {
+    /// Unknown, unmapped layout. Ghostty should not make any assumptions
+    /// about the layout of the keyboard.
+    Unknown,
+
+    // The remaining should be fairly self-explanatory:
+    UsStandard,
+    UsInternational,
+}
+
+impl Layout {
+    /// Map an Apple keyboard layout ID to a value in this enum. The layout
+    /// ID can be retrieved using Carbon's TIKeyboardLayoutGetInputSourceProperty
+    /// function.
+    ///
+    /// Even though our layout supports "unknown", we return None if we don't
+    /// recognize the layout ID so callers can detect this scenario.
+    pub fn map_apple_id(id: &str) -> Option<Layout> {
+        match id {
+            "com.apple.keylayout.US" => Some(Layout::UsStandard),
+            "com.apple.keylayout.USInternational" => Some(Layout::UsInternational),
+            _ => None,
+        }
+    }
+
+    /// Returns the default macos-option-as-alt value for this layout.
+    ///
+    /// We apply some heuristics to change the default based on the keyboard
+    /// layout if "macos-option-as-alt" is unset. We do this because on some
+    /// keyboard layouts such as US standard layouts, users generally expect
+    /// an input such as option-b to map to alt-b but macOS by default will
+    /// convert it to the codepoint "∫".
+    ///
+    /// This behavior however is desired on international layout where the
+    /// option key is used for important, regularly used inputs.
+    pub fn detect_option_as_alt(self) -> OptionAsAlt {
+        match self {
+            // On US standard, the option key is typically used as alt
+            // and not as a modifier for other codepoints. For example,
+            // option-B = ∫ but usually the user wants alt-B.
+            Layout::UsStandard | Layout::UsInternational => OptionAsAlt::True,
+            Layout::Unknown => OptionAsAlt::False,
+        }
+    }
+}