feat: port GTK version fallbacks, helpgen, bench modes, CLI leftovers

- apprt-gtk: Ghostty's runtime+build-time version gating (at_least)
  with cargo features adw_1_3/adw_1_4/adw_1_5/gtk_4_16 (default on) and
  the fallbacks for older libraries: MessageDialog (adw < 1.5),
  clipboard dialog without SwitchRow, pre-Banner debug warning and
  child-exited widgets, the old about dialog, GTK 4.16 CSS error source
  bytes. The minimum stays GTK 4.14 / libadwaita 1.5 (Ghostty's floor).
  The root build links with -z lazy so version-gated symbols don't keep
  older libraries from loading.
- ghostty-helpgen (port of helpgen.zig) generates CLI action help and
  keybinding action docs from doc comments at build time, replacing the
  checked-in help/*.txt; +help and +list-actions --docs stay
  byte-identical to Ghostty 1.3.1.
- ghostty-bench: libc wcwidth mode and separate simd/table and
  uucode/table code paths like Zig.
- ghostty-cli: Windows branches of edit-config, list-colors,
  list-keybinds and boo; ssh-cache DiskCache flock locking (no-op on
  Windows, as in Zig) with an exclusivity test.

Test Plan:
- cargo test --workspace
- diff <(cargo run -q -- +list-actions --docs) \
  <(ghostty +list-actions --docs)

crates/ghostty-apprt-gtk/src/class/window.rs

@@ -1657,18 +1657,29 @@ impl Window {
         let icon = "com.mitchellh.ghostty";
         let website = "https://ghostty.org";
 
-        // libadwaita >= 1.5 is assumed, so AdwAboutDialog is always used
-        // (the Zig GtkAboutDialog fallback is unreachable here).
-        let _ = adw_version::supports_dialogs();
-        let dialog = adw::AboutDialog::builder()
-            .application_name(name)
-            .developer_name(tr("Ghostty Developers"))
-            .application_icon(icon)
-            .version(env!("CARGO_PKG_VERSION"))
-            .issue_url("https://github.com/ghostty-org/ghostty/issues")
-            .website(website)
-            .build();
-        dialog.present(Some(self));
+        if adw_version::supports_dialogs() {
+            let dialog = adw::AboutDialog::builder()
+                .application_name(name)
+                .developer_name(tr("Ghostty Developers"))
+                .application_icon(icon)
+                .version(env!("CARGO_PKG_VERSION"))
+                .issue_url("https://github.com/ghostty-org/ghostty/issues")
+                .website(website)
+                .build();
+            dialog.present(Some(self));
+        } else {
+            // libadwaita < 1.5 has no AdwAboutDialog: use the GTK one.
+            gtk::show_about_dialog(
+                Some(self),
+                &[
+                    ("program-name", &name),
+                    ("logo-icon-name", &icon),
+                    ("title", &tr("About Ghostty")),
+                    ("version", &env!("CARGO_PKG_VERSION")),
+                    ("website", &website),
+                ],
+            );
+        }
     }
 
     fn action_close_tab(&self, param: Option<&glib::Variant>) {