Mind exposes its identity: the genesis receipt id

The instance name is only a label; two stores may share it. The receipt of the
first admission never changes and digests the first batch.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/huginn-mind/src/admission.rs

@@ -680,6 +680,19 @@ mod tests {
         assert!(Mind::open_with(store, &slug(OTHER_INSTANCE)).is_err());
     }
 
+    /// The identity the index labels its collection with: none before the
+    /// first write, then the first receipt's id and never a later one.
+    #[test]
+    fn the_genesis_receipt_id_is_the_first_admissions_and_stays_put() {
+        let mut mind = opened(MemoryStore::new(), INSTANCE);
+        assert_eq!(mind.genesis_receipt_id().unwrap(), None);
+        let (first, _) = committed(admit(&mut mind, vec![instance(INSTANCE), stewardship(INSTANCE, REPO)]));
+        assert_eq!(mind.genesis_receipt_id().unwrap(), Some(first.clone()));
+        let (second, _) = committed(admit(&mut mind, vec![campaign(&[REPO])]));
+        assert_ne!(first, second);
+        assert_eq!(mind.genesis_receipt_id().unwrap(), Some(first));
+    }
+
     #[test]
     fn admission_refuses_a_foreign_instance_whatever_the_transport() {
         let store = MemoryStore::new();

crates/huginn-mind/src/mind.rs

@@ -245,6 +245,16 @@ impl<S: MindStore> Mind<S> {
             .collect()
     }
 
+    /// The mind's own identity: the receipt id of its first admission
+    /// (ordinal 1), which admission never rewrites and which digests the
+    /// instance and the first batch's bytes. `None` for a mind that has not
+    /// been written yet. The instance name is only a label, since two stores
+    /// on two hosts may share it; this tells them apart unless their first
+    /// batches were byte-identical.
+    pub fn genesis_receipt_id(&self) -> Result<Option<String>, MindRefusal> {
+        Ok(self.receipts()?.into_iter().find(|receipt| receipt.ordinal == 1).map(|receipt| receipt.receipt_id))
+    }
+
     pub(crate) fn raw_envelope(&self, type_id: &str, key: &str) -> Option<&CultCacheEnvelope> {
         self.image.iter().find(|envelope| envelope.r#type == type_id && envelope.key == key)
     }