feat: honor async-backend with epoll/io_uring, fix last RV13 items

Resolves the 4 P3 items in docs/rewrite/notes/review-rv13-codex.md
(final-Z16.md):
- async-backend now takes effect like Ghostty: ghostty_os::xev ports
  libxev's backend detection (io_uring, then epoll); global init detects
  and logs it, the GTK startup applies the config with Zig's messages,
  and +version reports the real backend. The termio IO loop runs on a
  new reactor with epoll, io_uring (io-uring crate) and poll(2) (kept
  for macOS, where libxev uses kqueue). The 21 exec integration tests
  run under every backend; io_uring was exercised here.
- auto-update-channel defaults to the build's release channel.
- Windows error names: sharing violation -> AccessDenied, plus
  ERROR_FILE_EXISTS -> PathAlreadyExists and virus errors ->
  AntivirusInterference, mirroring std.os.windows.OpenFile's NTSTATUS
  mapping in Zig 0.15.2 (verified against its source by the
  orchestrator).
- Exact ports of std.fs.path.basenameWindows/basenamePosix for theme
  name validation (drive, UNC and device roots).

Test Plan:
- cargo test --workspace --no-fail-fast (3555 passed, exit 0)
- just check-macos (both), just check-windows, just check-wasm
- +show-config --default --docs vs /usr/bin/ghostty (only the
  build-specific gtk-opengl-debug line differs)

crates/ghostty-cli/src/version.rs

@@ -48,8 +48,7 @@ pub fn write_version(out: &mut dyn Write, info: &BuildInfo, tty: bool) -> std::i
     writeln!(out, "  - app runtime   : {}", info.app_runtime)?;
     writeln!(out, "  - font engine   : {}", info.font_backend)?;
     writeln!(out, "  - renderer      : {}", info.renderer)?;
-    // There is no libxev in the Rust port.
-    writeln!(out, "  - libxev        : n/a")?;
+    writeln!(out, "  - libxev        : {}", ghostty_os::xev::backend())?;
     if info.app_runtime == ".gtk" {
         let kernel_info = ghostty_os::get_kernel_info();
         writeln!(