fix-deviant(cli): never lose or overwrite CLI output like Zig does

Intentional deviation from Ghostty 1.3.1 (upstream data-loss bugs, see
docs/rewrite/DEVIANT_FIXES.md and docs/rewrite/notes/final-Z17.md):
- Zig 0.15's File.writer writes positionally (pwrite) at each writer's
  own offset, starting at 0, while the file is seekable. When stdout is
  a regular file, two writers (e.g. +show-face) overwrite each other and
  any existing content, and File.Writer.end truncates. ZigWriter always
  appends at the current end and end() only flushes.
- Writers that are never flushed lose their buffered output in Zig, e.g.
  +list-themes' stderr buffer, so "No themes found..." is never printed.
  ZigWriter flushes on drop.
Zig's buffering and flush order (and thus pipe/tty interleaving) is kept
unchanged; only the data loss is not replicated.

Test Plan:
- cargo test -p ghostty-cli zig_writer
  (regular_file_output_is_complete_and_appended,
  unflushed_buffer_is_flushed_on_drop)
- ghostty +list-themes with a missing resources dir prints the message

docs/rewrite/notes/final-Z17.md

@@ -0,0 +1,30 @@
+# Z17: RV14 findings
+
+1. `+show-face --cp` is `Option<U21>` (new `ghostty_config::cli_args::U21`, Zig `u21`); out of range gives an
+   `invalid value` diagnostic and exit 1 (no panic). Audit of Zig int-typed CLI/config fields against Rust
+   (Config.zig, cli/*.zig, nested types): all other widths already match.
+2. `--string` is `Option<EncodedOs>` (raw bytes at option parse). Invalid UTF-8 is detected after the `--cp`
+   lookup: `Unable to parse string as unicode: error.InvalidUtf8` (no newline), exit 1.
+3. New `crates/ghostty-cli/src/zig_writer.rs` models `File.writer(&buf)`: fixed buffer, drain-on-overflow,
+   explicit flush, so stdout/stderr interleave in Zig's flush order (`+show-face` creates both writers on
+   stdout; stdout is flushed first, then stderr; verified against the oracle on pipes). Anything still
+   buffered is flushed on drop. Applied to every Zig action using `File.writer`: show-face, help (+ any
+   `--help`), version, validate-config, show-config, list-actions, list-keybinds (plain), list-fonts,
+   list-themes, ssh-cache, crash-report, edit-config, new-window.
+4. Windows basename: already Zig 0.15.2 (colon only at byte index 1); `λ:theme` stays whole. Added test.
+5. `ghostty_os::osbytes::zig_is_absolute{,_posix,_windows}` + `EncodedOs::is_zig_absolute` (0.15.2
+   `isAbsolute`: `\x`, `/x`, `X:\`, `X:/`). Used by theme open, config path expand, file_load, Config
+   asserts, surface mouse path, GTK bell path (every Zig `isAbsolute` site).
+6. `xev::available(IoUring)` is exactly `IoUring::new(256).is_ok()`. The termio reactor now handles a ring
+   without `EXT_ARG` by using `IORING_OP_TIMEOUT` (like libxev timers); test forces that path.
+
+## Upstream bugs fixed (not replicated)
+
+- Positional output overwrite: `std.fs.File.writer` (Zig 0.15.2 `fs/File.zig` `Writer.init`/`drain`)
+  `pwrite`s at the writer's own offset starting at 0. With stdout a regular file, `+show-face`
+  (`cli/show_face.zig:68-73`, two writers on `File.stdout()`) overwrites its own output, and any action
+  (`cli/help.zig:34`, `cli/validate_config.zig:43`, ...) overwrites pre-existing file content; `Writer.end`
+  (`validate_config.zig:46`, `show_config.zig:84`) also truncates it. Rust always appends at the current end.
+- Never-flushed buffer: `cli/list_themes.zig:124-126,129-131,171` writes "Could not find the Ghostty
+  resources directory" / "No themes found..." to a 4096 byte stderr buffer that is never flushed, so the
+  messages are lost. Rust flushes it when the action ends.