Add metadata stripping, watermark positioning, and rename template modules

Metadata: JPEG EXIF stripping (APP1 segment removal).
Watermark: 9-position grid calculation with margin support.
Rename: template parser with {name}, {ext}, {counter:N}, {width}, {height}
  and collision resolution with auto-suffix.
Phase 4 complete - 79 tests passing, zero clippy warnings.

pixstrip-core/tests/metadata_tests.rs

@@ -0,0 +1,44 @@
+use pixstrip_core::operations::MetadataConfig;
+use pixstrip_core::operations::metadata::strip_metadata;
+use std::path::Path;
+
+fn create_test_jpeg(path: &Path) {
+    let img = image::RgbImage::from_fn(100, 80, |x, y| {
+        image::Rgb([(x % 256) as u8, (y % 256) as u8, 128])
+    });
+    img.save_with_format(path, image::ImageFormat::Jpeg).unwrap();
+}
+
+#[test]
+fn strip_all_metadata_produces_file() {
+    let dir = tempfile::tempdir().unwrap();
+    let input = dir.path().join("test.jpg");
+    let output = dir.path().join("stripped.jpg");
+    create_test_jpeg(&input);
+
+    strip_metadata(&input, &output, &MetadataConfig::StripAll).unwrap();
+    assert!(output.exists());
+    assert!(std::fs::metadata(&output).unwrap().len() > 0);
+}
+
+#[test]
+fn keep_all_metadata_copies_file() {
+    let dir = tempfile::tempdir().unwrap();
+    let input = dir.path().join("test.jpg");
+    let output = dir.path().join("kept.jpg");
+    create_test_jpeg(&input);
+
+    strip_metadata(&input, &output, &MetadataConfig::KeepAll).unwrap();
+    assert!(output.exists());
+}
+
+#[test]
+fn privacy_mode_strips_gps() {
+    let dir = tempfile::tempdir().unwrap();
+    let input = dir.path().join("test.jpg");
+    let output = dir.path().join("privacy.jpg");
+    create_test_jpeg(&input);
+
+    strip_metadata(&input, &output, &MetadataConfig::Privacy).unwrap();
+    assert!(output.exists());
+}