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)

Cargo.lock

@@ -1449,6 +1449,7 @@ name = "ghostty-config"
 version = "0.1.0"
 dependencies = [
  "const_format",
+ "ghostty-build-info",
  "ghostty-input",
  "ghostty-os",
  "ghostty-terminal",
@@ -1621,6 +1622,7 @@ dependencies = [
  "gettext-rs",
  "ghostty-vt",
  "ghostty-zigfmt",
+ "io-uring",
  "libc",
  "log",
  "nix 0.31.3",
@@ -1723,6 +1725,7 @@ dependencies = [
  "ghostty-renderer",
  "ghostty-terminal",
  "ghostty-vt",
+ "io-uring",
  "libc",
  "log",
  "nix 0.31.3",
@@ -2150,6 +2153,17 @@ dependencies = [
  "syn 3.0.8",
 ]
 
+[[package]]
+name = "io-uring"
+version = "0.7.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ed3bd0ecfbb87805f538bb7b32e5239ca0763890c623e349860ecba69469f2bb"
+dependencies = [
+ "bitflags 2.13.2",
+ "cfg-if",
+ "libc",
+]
+
 [[package]]
 name = "is_terminal_polyfill"
 version = "1.70.2"