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/Cargo.toml

@@ -5,6 +5,18 @@ edition.workspace = true
 license.workspace = true
 build = "build.rs"
 
+[features]
+# Build-time ("header") capability levels, the Rust counterpart of the C
+# headers' version macros that Zig's `gtk_version.atLeast`/`adw_version.atLeast`
+# check at comptime. Every `atLeast` is "build level >= requested AND runtime
+# library >= requested", so with a lower build level the code that needs newer
+# API (and the Zig compatibility fallbacks) is compiled instead.
+default = ["adw_1_5", "gtk_4_16"]
+adw_1_3 = []
+adw_1_4 = ["adw_1_3"]
+adw_1_5 = ["adw_1_4"]
+gtk_4_16 = ["gtk/v4_16"]
+
 [dependencies]
 ghostty-apprt.workspace = true
 ghostty-config.workspace = true
@@ -16,7 +28,7 @@ ghostty-vt.workspace = true
 log = "0.4"
 thiserror = "2"
 bitflags = "2"
-gtk = { package = "gtk4", version = "0.11", features = ["v4_14", "gnome_44"] }
+gtk = { package = "gtk4", version = "0.11", features = ["v4_14"] }
 adw = { package = "libadwaita", version = "0.9", features = ["v1_5"] }
 gtk4-layer-shell = "0.8"
 gdk4-x11 = { version = "0.11.5", features = ["x11", "xlib"] }