[fix](trx-wefax): finalize and fsync PNG file before reporting path

Relying on Drop to write the PNG IEND trailer and flush BufWriter
silently swallows I/O errors, which can leave truncated/corrupted
image files on disk. Explicitly call writer.finish() to surface
encoding errors and sync_all() the File so bytes are durable before
WefaxEvent::Complete is emitted with the path (the frontend may read
the file immediately). The intermediate BufWriter is dropped since
we already buffer all rows and write them in a single call.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/decoders/trx-wefax/src/image.rs

@@ -4,7 +4,7 @@
 
 //! Image buffer and PNG encoding for WEFAX decoded images.
 
-use std::io::BufWriter;
+use std::io::Write;
 use std::path::{Path, PathBuf};
 
 /// Image assembler: accumulates greyscale lines and encodes to PNG.
@@ -91,14 +91,16 @@ impl ImageAssembler {
         let filename = generate_filename(freq_hz, mode);
         let path = output_dir.join(&filename);
 
+        // We already buffer the image rows into `img_data` below and
+        // write them in a single call, so a BufWriter adds no value.
+        // Using the bare `File` also lets us fsync explicitly below.
         let file = std::fs::File::create(&path)
             .map_err(|e| format!("create PNG file '{}': {}", path.display(), e))?;
-        let w = BufWriter::new(file);
 
         let width = self.pixels_per_line as u32;
         let height = self.lines.len() as u32;
 
-        let mut encoder = png::Encoder::new(w, width, height);
+        let mut encoder = png::Encoder::new(&file, width, height);
         encoder.set_color(png::ColorType::Grayscale);
         encoder.set_depth(png::BitDepth::Eight);
 
@@ -116,6 +118,19 @@ impl ImageAssembler {
             .write_image_data(&img_data)
             .map_err(|e| format!("write PNG data: {}", e))?;
 
+        // Explicitly finish the writer (writes IEND). Relying on Drop
+        // alone swallows any I/O error and can yield a truncated file.
+        writer
+            .finish()
+            .map_err(|e| format!("finalize PNG: {}", e))?;
+        // Flush the underlying file so the data is durably on disk by
+        // the time we emit the WefaxEvent::Complete.
+        (&file)
+            .flush()
+            .map_err(|e| format!("flush PNG file: {}", e))?;
+        file.sync_all()
+            .map_err(|e| format!("sync PNG file: {}", e))?;
+
         Ok(path)
     }