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

crates/ghostty-cli/src/list_themes.rs

@@ -233,6 +233,8 @@ pub fn run(args: &[impl AsRef<[u8]>]) -> Result<u8, Error> {
     crate::parse_options(&mut opts, args)?;
 
     let stdout = std::io::stdout();
+    // Zig's stderr is a 4096 byte buffer that it never flushes (the messages
+    // below are lost). That is an upstream bug: we flush when it is dropped.
     let mut stderr = crate::zig_writer::ZigWriter::new(crate::zig_writer::stderr_sink(), 4096);
 
     let resources_dir = resources_dir();

crates/ghostty-cli/src/zig_writer.rs

@@ -4,6 +4,13 @@
 //! file when it overflows or when the action calls `flush`, so two writers on
 //! the same file (as in `+show-face`) interleave in flush order, not call
 //! order. This type keeps that ordering.
+//!
+//! Two upstream data-loss behaviours are deliberately NOT replicated (see
+//! `docs/rewrite/notes/final-Z17.md`): Zig's positional `pwrite` at the
+//! writer's own offset 0 (writers overwrite each other and existing content
+//! when stdout is a regular file) and buffers that are never flushed. Data is
+//! always appended at the current end and anything still buffered is flushed
+//! when the writer is dropped.
 
 #[cfg(unix)]
 use std::os::fd::AsFd;
@@ -69,7 +76,7 @@ impl ZigWriter {
         }
     }
 
-    /// `File.Writer.end`.
+    /// `File.Writer.end`: a flush (Zig also truncates; not replicated).
     pub fn end(&mut self) -> io::Result<()> {
         self.flush()
     }
@@ -91,6 +98,12 @@ impl ZigWriter {
     }
 }
 
+impl Drop for ZigWriter {
+    fn drop(&mut self) {
+        let _ = self.flush();
+    }
+}
+
 impl Write for ZigWriter {
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         self.write_all(data)?;
@@ -154,4 +167,32 @@ mod tests {
         w.write_all(b"h").unwrap();
         assert_eq!(&*mem.0.borrow(), b"abcdefg");
     }
+
+    #[cfg(unix)]
+    #[test]
+    fn regular_file_output_is_complete_and_appended() {
+        let mut tmp = tempfile::tempfile().unwrap();
+        std::io::Write::write_all(&mut tmp, b"pre\n").unwrap();
+        let sink: Sink = Rc::new(RefCell::new(Target::Fd(tmp.try_clone().unwrap())));
+        let mut a = ZigWriter::new(sink.clone(), 1024);
+        let mut b = ZigWriter::new(sink, 1024);
+        a.write_all(b"0123456789\n").unwrap();
+        b.write_all(b"abc\n").unwrap();
+        a.flush().unwrap();
+        b.flush().unwrap();
+        let mut out = String::new();
+        std::io::Seek::rewind(&mut tmp).unwrap();
+        std::io::Read::read_to_string(&mut tmp, &mut out).unwrap();
+        assert_eq!(out, "pre\n0123456789\nabc\n");
+    }
+
+    #[test]
+    fn unflushed_buffer_is_flushed_on_drop() {
+        let mem = Mem::default();
+        let sink: Sink = stream_sink(mem.clone());
+        let mut w = ZigWriter::new(sink, 4096);
+        w.write_all(b"kept").unwrap();
+        drop(w);
+        assert_eq!(&*mem.0.borrow(), b"kept");
+    }
 }