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-core/src/global.rs

@@ -204,6 +204,19 @@ fn init_with(opts: InitOptions, default_logging: Logging) -> &'static GlobalStat
         // SIGPIPE is ignored by the Rust runtime already (it sets SIG_IGN
         // before main), which is what Zig's `initSignals` does.
 
+        // Setup our Xev backend if we're dynamic
+        if ghostty_os::xev::is_dynamic()
+            && let Err(err) = ghostty_os::xev::detect()
+        {
+            log::warn!(
+                "failed to detect xev backend, falling back to most compatible backend err={err}"
+            );
+        }
+        log::info!(
+            "libxev default backend={}",
+            ghostty_os::xev::backend().name()
+        );
+
         // As early as possible, initialize our resource limits.
         let rlimits = ResourceLimits::init();