Fix 26 bugs, edge cases, and consistency issues from fifth audit pass

Critical: undo toast now trashes only batch output files (not entire dir),
JPEG scanline write errors propagated, selective metadata write result returned.

High: zero-dimension guards in ResizeConfig/fit_within, negative aspect ratio
rejection, FM integration toggle infinite recursion guard, saturating counter
arithmetic in executor.

Medium: PNG compression level passed to oxipng, pct mode updates job_config,
external file loading updates step indicator, CLI undo removes history entries,
watch config write failures reported, fast-copy path reads image dimensions for
rename templates, discovery excludes unprocessable formats (heic/svg/ico/jxl),
CLI warns on invalid algorithm/overwrite values, resolve_collision trailing dot
fix, generation guards on all preview threads to cancel stale results, default
DPI aligned to 0, watermark text width uses char count not byte length.

Low: binary path escaped in Nautilus extension, file dialog filter aligned with
discovery, reset_wizard clears preset_mode and output_dir.

pixstrip-core/tests/watermark_tests.rs

@@ -109,3 +109,137 @@ fn position_bottom_left() {
     assert_eq!(x, 10);
     assert_eq!(y, 1020);
 }
+
+// --- Margin variation tests ---
+
+#[test]
+fn margin_zero_top_left() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::TopLeft,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        0,
+    );
+    assert_eq!(x, 0);
+    assert_eq!(y, 0);
+}
+
+#[test]
+fn margin_zero_bottom_right() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomRight,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        0,
+    );
+    assert_eq!(x, 1720); // 1920 - 200
+    assert_eq!(y, 1030); // 1080 - 50
+}
+
+#[test]
+fn large_margin_top_left() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::TopLeft,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        100,
+    );
+    assert_eq!(x, 100);
+    assert_eq!(y, 100);
+}
+
+#[test]
+fn large_margin_bottom_right() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomRight,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        100,
+    );
+    assert_eq!(x, 1620); // 1920 - 200 - 100
+    assert_eq!(y, 930);  // 1080 - 50 - 100
+}
+
+#[test]
+fn margin_does_not_affect_center() {
+    let (x1, y1) = calculate_position(
+        WatermarkPosition::Center,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        0,
+    );
+    let (x2, y2) = calculate_position(
+        WatermarkPosition::Center,
+        Dimensions { width: 1920, height: 1080 },
+        Dimensions { width: 200, height: 50 },
+        100,
+    );
+    assert_eq!(x1, x2);
+    assert_eq!(y1, y2);
+}
+
+// --- Edge case tests ---
+
+#[test]
+fn watermark_larger_than_image() {
+    // Watermark is bigger than the image - should not panic, saturating_sub clamps to 0
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomRight,
+        Dimensions { width: 100, height: 100 },
+        Dimensions { width: 200, height: 200 },
+        10,
+    );
+    // (100 - 200 - 10) saturates to 0
+    assert_eq!(x, 0);
+    assert_eq!(y, 0);
+}
+
+#[test]
+fn watermark_exact_image_size() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::Center,
+        Dimensions { width: 200, height: 100 },
+        Dimensions { width: 200, height: 100 },
+        0,
+    );
+    assert_eq!(x, 0);
+    assert_eq!(y, 0);
+}
+
+#[test]
+fn zero_size_image() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::Center,
+        Dimensions { width: 0, height: 0 },
+        Dimensions { width: 200, height: 50 },
+        10,
+    );
+    assert_eq!(x, 0);
+    assert_eq!(y, 0);
+}
+
+#[test]
+fn margin_exceeds_available_space() {
+    // Margin is huge relative to image size
+    let (x, y) = calculate_position(
+        WatermarkPosition::BottomRight,
+        Dimensions { width: 100, height: 100 },
+        Dimensions { width: 50, height: 50 },
+        200,
+    );
+    // saturating_sub: 100 - 50 - 200 = 0
+    assert_eq!(x, 0);
+    assert_eq!(y, 0);
+}
+
+#[test]
+fn one_pixel_image() {
+    let (x, y) = calculate_position(
+        WatermarkPosition::TopLeft,
+        Dimensions { width: 1, height: 1 },
+        Dimensions { width: 1, height: 1 },
+        0,
+    );
+    assert_eq!(x, 0);
+    assert_eq!(y, 0);
+}