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/link.rs

@@ -0,0 +1,63 @@
+//! A link is a clickable element that can be used to trigger some action.
+//! A link is NOT just a URL that opens in a browser. A link is any generic
+//! regular expression match over terminal text that can trigger various
+//! action types. (ZIG/src/input/Link.zig)
+
+use crate::key_mods::Mods;
+
+/// The action that will be triggered when the link is clicked.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Action {
+    /// Open the full matched value using the default open program.
+    /// For example, on macOS this is "open" and on Linux this is "xdg-open".
+    Open,
+
+    /// Open the OSC8 hyperlink under the mouse position. _-prefixed means
+    /// this can't be user-specified, it's only used internally.
+    _OpenOsc8,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Highlight {
+    /// Always highlight the link.
+    Always,
+
+    /// Only highlight the link when the mouse is hovering over it.
+    Hover,
+
+    /// Highlight anytime the given mods are pressed, either when
+    /// hovering or always. For always, all links will be highlighted
+    /// when the mods are pressed regardless of if the mouse is hovering
+    /// over them.
+    ///
+    /// Note that if "shift" is specified here, this will NEVER match in
+    /// TUI programs that capture mouse events. "Shift" with mouse capture
+    /// escapes the mouse capture but strips the "shift" so it can't be
+    /// detected.
+    AlwaysMods(Mods),
+    HoverMods(Mods),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Link {
+    /// The regular expression that will be used to match the link.
+    pub regex: String,
+
+    /// The action that will be triggered when the link is clicked.
+    pub action: Action,
+
+    /// The situations in which the link will be highlighted. A link is only
+    /// clickable by the mouse when it is highlighted, so this also controls
+    /// when the link is clickable.
+    pub highlight: Highlight,
+}
+
+// TODO(integration): `oni_regex` (Zig `oniRegex`) builds an Oniguruma regex;
+// the regex engine choice is made by the consumer crate (config/core).
+
+impl Link {
+    /// Check if two links are equal.
+    pub fn equal(&self, other: &Link) -> bool {
+        self == other
+    }
+}