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)

build.rs

@@ -0,0 +1,16 @@
+//! Link the final binary with lazy symbol binding.
+//!
+//! Rust links executables with `-z now` (eager binding) by default. The GTK
+//! apprt compiles in code for newer GTK/libadwaita API (e.g. the GTK 4.16 CSS
+//! diagnostics) that is only ever called after a runtime `at_least` check,
+//! exactly like the Zig build, which links GTK dynamically with lazy binding.
+//! With eager binding the dynamic loader would refuse to start the program on
+//! an older library that merely lacks such a symbol; with lazy binding the
+//! symbol is only resolved if the guarded code actually runs.
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    if std::env::var("CARGO_CFG_TARGET_OS").as_deref() == Ok("linux") {
+        println!("cargo:rustc-link-arg-bins=-Wl,-z,lazy");
+    }
+}