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());
+}

pixstrip-core/tests/rename_tests.rs

@@ -0,0 +1,95 @@
+use pixstrip_core::operations::rename::{apply_template, resolve_collision};
+
+#[test]
+fn template_basic_variables() {
+    let result = apply_template(
+        "{name}_{counter:3}.{ext}",
+        "sunset",
+        "jpg",
+        1,
+        None,
+    );
+    assert_eq!(result, "sunset_001.jpg");
+}
+
+#[test]
+fn template_with_prefix() {
+    let result = apply_template(
+        "blog_{name}.{ext}",
+        "photo",
+        "webp",
+        1,
+        None,
+    );
+    assert_eq!(result, "blog_photo.webp");
+}
+
+#[test]
+fn template_counter_padding() {
+    let result = apply_template(
+        "{name}_{counter:4}.{ext}",
+        "img",
+        "png",
+        42,
+        None,
+    );
+    assert_eq!(result, "img_0042.png");
+}
+
+#[test]
+fn template_counter_no_padding() {
+    let result = apply_template(
+        "{name}_{counter}.{ext}",
+        "img",
+        "png",
+        42,
+        None,
+    );
+    assert_eq!(result, "img_42.png");
+}
+
+#[test]
+fn template_width_height() {
+    let result = apply_template(
+        "{name}_{width}x{height}.{ext}",
+        "photo",
+        "jpg",
+        1,
+        Some((1920, 1080)),
+    );
+    assert_eq!(result, "photo_1920x1080.jpg");
+}
+
+#[test]
+fn collision_adds_suffix() {
+    let dir = tempfile::tempdir().unwrap();
+    let base = dir.path().join("photo.jpg");
+    std::fs::write(&base, b"exists").unwrap();
+
+    let resolved = resolve_collision(&base);
+    assert_eq!(
+        resolved.file_name().unwrap().to_str().unwrap(),
+        "photo_1.jpg"
+    );
+}
+
+#[test]
+fn collision_increments() {
+    let dir = tempfile::tempdir().unwrap();
+    std::fs::write(dir.path().join("photo.jpg"), b"exists").unwrap();
+    std::fs::write(dir.path().join("photo_1.jpg"), b"exists").unwrap();
+
+    let resolved = resolve_collision(&dir.path().join("photo.jpg"));
+    assert_eq!(
+        resolved.file_name().unwrap().to_str().unwrap(),
+        "photo_2.jpg"
+    );
+}
+
+#[test]
+fn no_collision_returns_same() {
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("unique.jpg");
+    let resolved = resolve_collision(&path);
+    assert_eq!(resolved, path);
+}

pixstrip-core/tests/watermark_tests.rs

@@ -0,0 +1,111 @@
+use pixstrip_core::operations::watermark::calculate_position;
+use pixstrip_core::operations::WatermarkPosition;
+use pixstrip_core::types::Dimensions;
+
+#[test]
+fn position_top_left() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::TopLeft,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 10);
+    assert_eq!(y, 10);
+}
+
+#[test]
+fn position_center() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::Center,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 860);  // (1920 - 200) / 2
+    assert_eq!(y, 515);  // (1080 - 50) / 2
+}
+
+#[test]
+fn position_bottom_right() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomRight,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 1710); // 1920 - 200 - 10
+    assert_eq!(y, 1020); // 1080 - 50 - 10
+}
+
+#[test]
+fn position_top_center() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::TopCenter,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 860); // (1920 - 200) / 2
+    assert_eq!(y, 10);
+}
+
+#[test]
+fn position_bottom_center() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomCenter,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 860);
+    assert_eq!(y, 1020);
+}
+
+#[test]
+fn position_middle_left() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::MiddleLeft,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 10);
+    assert_eq!(y, 515);
+}
+
+#[test]
+fn position_middle_right() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::MiddleRight,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 1710);
+    assert_eq!(y, 515);
+}
+
+#[test]
+fn position_top_right() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::TopRight,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 1710);
+    assert_eq!(y, 10);
+}
+
+#[test]
+fn position_bottom_left() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomLeft,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 10);
+    assert_eq!(y, 1020);
+}