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/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);
+}