feat: flip canonical type field — make type: primary, Is A: the alias

The Rust parser now treats `type:` as the canonical frontmatter field
for entity type, with `Is A:` and `is_a:` accepted as legacy aliases.
Previously it was the other way around, creating an asymmetric
read/write cycle since the frontend and all 8800+ vault notes already
use `type:`.

- Flip serde attribute: rename="type", alias="Is A", alias="is_a"
- Update theme defaults, getting-started vault, and type definitions
- Add round-trip tests for both type: and Is A: parsing
- Update mock data and TypeScript tests to use canonical form

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/theme/defaults.rs

@@ -164,7 +164,7 @@ pub const DEFAULT_VAULT_THEME_VARS: [(&str, &str); 46] = [
 
 /// Vault-based theme note for the built-in Default theme.
 pub const DEFAULT_VAULT_THEME: &str = "---\n\
-Is A: Theme\n\
+type: Theme\n\
 Description: Light theme with warm, paper-like tones\n\
 background: \"#FFFFFF\"\n\
 foreground: \"#37352F\"\n\
@@ -220,7 +220,7 @@ The default light theme for Laputa. Clean and warm, inspired by Notion.\n";
 
 /// Vault-based theme note for the built-in Dark theme.
 pub const DARK_VAULT_THEME: &str = "---\n\
-Is A: Theme\n\
+type: Theme\n\
 Description: Dark variant with deep navy tones\n\
 background: \"#0f0f1a\"\n\
 foreground: \"#e0e0e0\"\n\
@@ -276,7 +276,7 @@ A dark theme with deep navy tones for comfortable night-time reading.\n";
 
 /// Vault-based theme note for the built-in Minimal theme.
 pub const MINIMAL_VAULT_THEME: &str = "---\n\
-Is A: Theme\n\
+type: Theme\n\
 Description: High contrast, minimal chrome\n\
 background: \"#FAFAFA\"\n\
 foreground: \"#111111\"\n\
@@ -332,7 +332,7 @@ High contrast, minimal chrome. Monospace typography throughout.\n";
 
 /// Type definition for the Theme note type.
 pub const THEME_TYPE_DEFINITION: &str = "---\n\
-Is A: Type\n\
+type: Type\n\
 icon: palette\n\
 color: purple\n\
 order: 50\n\

src-tauri/src/theme/seed.rs

@@ -181,7 +181,7 @@ mod tests {
 
         seed_vault_themes(vp);
         let content = fs::read_to_string(theme_dir.join("default.md")).unwrap();
-        assert!(content.contains("Is A: Theme"));
+        assert!(content.contains("type: Theme"));
     }
 
     #[test]
@@ -190,7 +190,7 @@ mod tests {
         let vault = dir.path().join("vault");
         let theme_dir = vault.join("theme");
         fs::create_dir_all(&theme_dir).unwrap();
-        let custom = "---\nIs A: Theme\nbackground: \"#FF0000\"\n---\n# Custom\n";
+        let custom = "---\ntype: Theme\nbackground: \"#FF0000\"\n---\n# Custom\n";
         fs::write(theme_dir.join("default.md"), custom).unwrap();
         let vp = vault.to_str().unwrap();
 
@@ -227,7 +227,7 @@ mod tests {
 
         ensure_vault_themes(vp).unwrap();
         let content = fs::read_to_string(theme_dir.join("default.md")).unwrap();
-        assert!(content.contains("Is A: Theme"));
+        assert!(content.contains("type: Theme"));
     }
 
     #[test]
@@ -236,7 +236,7 @@ mod tests {
         let vault = dir.path().join("vault");
         let theme_dir = vault.join("theme");
         fs::create_dir_all(&theme_dir).unwrap();
-        let custom = "---\nIs A: Theme\nbackground: \"#123456\"\n---\n";
+        let custom = "---\ntype: Theme\nbackground: \"#123456\"\n---\n";
         fs::write(theme_dir.join("default.md"), custom).unwrap();
         let vp = vault.to_str().unwrap();
 
@@ -265,7 +265,7 @@ mod tests {
             "restore must create type/theme.md"
         );
         let type_content = fs::read_to_string(vault.join("type").join("theme.md")).unwrap();
-        assert!(type_content.contains("Is A: Type"));
+        assert!(type_content.contains("type: Type"));
         assert!(type_content.contains("icon: palette"));
     }
 
@@ -280,7 +280,7 @@ mod tests {
         let path = vault.join("type").join("theme.md");
         assert!(path.exists());
         let content = fs::read_to_string(&path).unwrap();
-        assert!(content.contains("Is A: Type"));
+        assert!(content.contains("type: Type"));
         assert!(content.contains("icon: palette"));
     }
 
@@ -290,7 +290,7 @@ mod tests {
         let vault = dir.path().join("vault");
         let type_dir = vault.join("type");
         fs::create_dir_all(&type_dir).unwrap();
-        let custom = "---\nIs A: Type\nicon: swatches\ncolor: green\n---\n# Theme\n";
+        let custom = "---\ntype: Type\nicon: swatches\ncolor: green\n---\n# Theme\n";
         fs::write(type_dir.join("theme.md"), custom).unwrap();
         let vp = vault.to_str().unwrap();