chore(core): add Default impls + FieldId uniqueness test

Code review fixups:
- ItemId/FieldId need impl Default delegating to ::new() to silence
  clippy::new_without_default
- FieldId was missing the parallel uniqueness test that ItemId has

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/ids.rs

@@ -31,6 +31,10 @@ impl ItemId {
     pub fn as_str(&self) -> &str { &self.0 }
 }
 
+impl Default for ItemId {
+    fn default() -> Self { Self::new() }
+}
+
 impl FieldId {
     pub fn new() -> Self {
         let mut bytes = [0u8; 8];
@@ -40,6 +44,10 @@ impl FieldId {
     pub fn as_str(&self) -> &str { &self.0 }
 }
 
+impl Default for FieldId {
+    fn default() -> Self { Self::new() }
+}
+
 impl AttachmentId {
     pub fn from_plaintext(plaintext: &[u8]) -> Self {
         let digest = Sha256::digest(plaintext);
@@ -74,6 +82,14 @@ mod tests {
         assert!(id.0.chars().all(|c| c.is_ascii_hexdigit()));
     }
 
+    #[test]
+    fn field_ids_are_unique() {
+        let mut seen = std::collections::HashSet::new();
+        for _ in 0..10_000 {
+            assert!(seen.insert(FieldId::new().0));
+        }
+    }
+
     #[test]
     fn attachment_id_is_deterministic() {
         let plaintext = b"hello world";