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/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);
+}