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/adjustments_tests.rs

@@ -0,0 +1,145 @@
+use pixstrip_core::operations::AdjustmentsConfig;
+use pixstrip_core::operations::adjustments::apply_adjustments;
+use image::DynamicImage;
+
+fn noop_config() -> AdjustmentsConfig {
+    AdjustmentsConfig {
+        brightness: 0,
+        contrast: 0,
+        saturation: 0,
+        sharpen: false,
+        grayscale: false,
+        sepia: false,
+        crop_aspect_ratio: None,
+        trim_whitespace: false,
+        canvas_padding: 0,
+    }
+}
+
+#[test]
+fn is_noop_default() {
+    assert!(noop_config().is_noop());
+}
+
+#[test]
+fn is_noop_with_brightness() {
+    let mut config = noop_config();
+    config.brightness = 10;
+    assert!(!config.is_noop());
+}
+
+#[test]
+fn is_noop_with_sharpen() {
+    let mut config = noop_config();
+    config.sharpen = true;
+    assert!(!config.is_noop());
+}
+
+#[test]
+fn is_noop_with_crop() {
+    let mut config = noop_config();
+    config.crop_aspect_ratio = Some((16.0, 9.0));
+    assert!(!config.is_noop());
+}
+
+#[test]
+fn is_noop_with_trim() {
+    let mut config = noop_config();
+    config.trim_whitespace = true;
+    assert!(!config.is_noop());
+}
+
+#[test]
+fn is_noop_with_padding() {
+    let mut config = noop_config();
+    config.canvas_padding = 20;
+    assert!(!config.is_noop());
+}
+
+#[test]
+fn is_noop_with_grayscale() {
+    let mut config = noop_config();
+    config.grayscale = true;
+    assert!(!config.is_noop());
+}
+
+#[test]
+fn is_noop_with_sepia() {
+    let mut config = noop_config();
+    config.sepia = true;
+    assert!(!config.is_noop());
+}
+
+// --- crop_to_aspect_ratio edge cases ---
+
+fn make_test_image(w: u32, h: u32) -> DynamicImage {
+    DynamicImage::ImageRgba8(image::RgbaImage::from_pixel(w, h, image::Rgba([128, 128, 128, 255])))
+}
+
+#[test]
+fn crop_zero_ratio_returns_original() {
+    let img = make_test_image(100, 100);
+    let config = AdjustmentsConfig {
+        crop_aspect_ratio: Some((0.0, 9.0)),
+        ..noop_config()
+    };
+    let result = apply_adjustments(img, &config).unwrap();
+    assert_eq!(result.width(), 100);
+    assert_eq!(result.height(), 100);
+}
+
+#[test]
+fn crop_zero_height_ratio_returns_original() {
+    let img = make_test_image(100, 100);
+    let config = AdjustmentsConfig {
+        crop_aspect_ratio: Some((16.0, 0.0)),
+        ..noop_config()
+    };
+    let result = apply_adjustments(img, &config).unwrap();
+    assert_eq!(result.width(), 100);
+    assert_eq!(result.height(), 100);
+}
+
+#[test]
+fn crop_square_on_landscape() {
+    let img = make_test_image(200, 100);
+    let config = AdjustmentsConfig {
+        crop_aspect_ratio: Some((1.0, 1.0)),
+        ..noop_config()
+    };
+    let result = apply_adjustments(img, &config).unwrap();
+    assert_eq!(result.width(), 100);
+    assert_eq!(result.height(), 100);
+}
+
+#[test]
+fn crop_16_9_on_square() {
+    let img = make_test_image(100, 100);
+    let config = AdjustmentsConfig {
+        crop_aspect_ratio: Some((16.0, 9.0)),
+        ..noop_config()
+    };
+    let result = apply_adjustments(img, &config).unwrap();
+    assert_eq!(result.width(), 100);
+    assert_eq!(result.height(), 56);
+}
+
+#[test]
+fn canvas_padding_large_value() {
+    let img = make_test_image(10, 10);
+    let config = AdjustmentsConfig {
+        canvas_padding: 500,
+        ..noop_config()
+    };
+    let result = apply_adjustments(img, &config).unwrap();
+    assert_eq!(result.width(), 1010);
+    assert_eq!(result.height(), 1010);
+}
+
+#[test]
+fn noop_config_returns_same_dimensions() {
+    let img = make_test_image(200, 100);
+    let result = apply_adjustments(img, &noop_config()).unwrap();
+    assert_eq!(result.width(), 200);
+    assert_eq!(result.height(), 100);
+}