Fix 40+ bugs from audit passes 9-12

- PNG chunk parsing overflow protection with checked arithmetic
- Font directory traversal bounded with global result limit
- find_unique_path TOCTOU race fixed with create_new + marker byte
- Watch mode "processed" dir exclusion narrowed to prevent false skips
- Metadata copy now checks format support before little_exif calls
- Clipboard temp files cleaned up on app exit
- Atomic writes for file manager integration scripts
- BMP format support added to encoder and convert step
- Regex DoS protection with DFA size limit
- Watermark NaN/negative scale guard
- Selective EXIF stripping for privacy/custom metadata modes
- CLI watch mode: file stability checks, per-file history saves
- High contrast toggle preserves and restores original theme
- Image list deduplication uses O(1) HashSet lookups
- Saturation/trim/padding overflow guards in adjustments

pixstrip-core/src/storage.rs

@@ -14,7 +14,7 @@ fn default_config_dir() -> PathBuf {
 }
 
 /// Write to a temporary file then rename, for crash safety.
-fn atomic_write(path: &Path, contents: &str) -> std::io::Result<()> {
+pub fn atomic_write(path: &Path, contents: &str) -> std::io::Result<()> {
     let tmp = path.with_extension("tmp");
     std::fs::write(&tmp, contents)?;
     std::fs::rename(&tmp, path)?;