Add Preset type with 8 built-in presets and JSON serialization

All 5 preset tests passing.

pixstrip-core/tests/preset_tests.rs

@@ -0,0 +1,59 @@
+use pixstrip_core::preset::*;
+use pixstrip_core::operations::*;
+
+#[test]
+fn builtin_preset_blog_photos() {
+    let preset = Preset::builtin_blog_photos();
+    assert_eq!(preset.name, "Blog Photos");
+    assert!(preset.resize.is_some());
+    assert!(preset.compress.is_some());
+    assert!(preset.metadata.is_some());
+    assert!(!preset.is_custom);
+}
+
+#[test]
+fn builtin_preset_privacy_clean() {
+    let preset = Preset::builtin_privacy_clean();
+    assert_eq!(preset.name, "Privacy Clean");
+    assert!(preset.resize.is_none());
+    assert!(preset.compress.is_none());
+    assert!(preset.metadata.is_some());
+    if let Some(MetadataConfig::StripAll) = &preset.metadata {
+        // correct
+    } else {
+        panic!("Expected StripAll metadata config");
+    }
+}
+
+#[test]
+fn preset_serialization_roundtrip() {
+    let preset = Preset::builtin_blog_photos();
+    let json = serde_json::to_string_pretty(&preset).unwrap();
+    let deserialized: Preset = serde_json::from_str(&json).unwrap();
+    assert_eq!(deserialized.name, preset.name);
+    assert_eq!(deserialized.is_custom, preset.is_custom);
+}
+
+#[test]
+fn all_builtin_presets() {
+    let presets = Preset::all_builtins();
+    assert_eq!(presets.len(), 8);
+    let names: Vec<&str> = presets.iter().map(|p| p.name.as_str()).collect();
+    assert!(names.contains(&"Blog Photos"));
+    assert!(names.contains(&"Social Media"));
+    assert!(names.contains(&"Web Optimization"));
+    assert!(names.contains(&"Email Friendly"));
+    assert!(names.contains(&"Privacy Clean"));
+    assert!(names.contains(&"Photographer Export"));
+    assert!(names.contains(&"Archive Compress"));
+    assert!(names.contains(&"Fediverse Ready"));
+}
+
+#[test]
+fn preset_to_processing_job() {
+    let preset = Preset::builtin_blog_photos();
+    let job = preset.to_job("/tmp/input/", "/tmp/output/");
+    assert!(job.resize.is_some());
+    assert!(job.compress.is_some());
+    assert!(job.metadata.is_some());
+}