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/executor_tests.rs

@@ -112,9 +112,14 @@ fn execute_with_cancellation() {
     let executor = PipelineExecutor::with_cancel(cancel);
     let result = executor.execute(&job, |_| {}).unwrap();
 
-    // With immediate cancellation, fewer images should be processed
-    assert!(result.succeeded + result.failed <= 2);
-    assert!(result.cancelled);
+    // Cancellation flag should be set
+    assert!(result.cancelled, "result.cancelled should be true when cancel flag is set");
+    // Total processed should be less than total sources (at least some skipped)
+    assert!(
+        result.succeeded + result.failed <= 2,
+        "processed count ({}) should not exceed total (2)",
+        result.succeeded + result.failed
+    );
 }
 
 #[test]