refactor: CogRecord to fixed [Container; 2] = 2KB layout

The "holy grail" change: CogRecord is now exactly 2KB (meta + content),
stack-allocated, #[repr(C, align(64))]. Multi-container geometries use
linked records in the DN tree instead of Vec<Container>.

- Contract: CogRecord { meta: Container, content: Container }
- Added as_bytes()/from_bytes() for zero-copy I/O
- Updated all callers: geometry, dn_redis, graph, traversal, migrate
- Converted orphan impls to extension traits (CogRecordSerde)
- All 729 tests pass

https://claude.ai/code/session_01NocTBszN8LoR9Vrc7Tn4oF

Author: claude

crates/ladybug-contract/src/delegation.rs

@@ -40,17 +40,15 @@ mod cogrecord_serde {
     pub fn serialize<S: Serializer>(record: &CogRecord, serializer: S) -> Result<S::Ok, S::Error> {
         let mut s = serializer.serialize_struct("CogRecord", 2)?;
         s.serialize_field("meta", &record.meta.words.as_slice())?;
-        let content_words: Vec<&[u64]> =
-            record.content.iter().map(|c| c.words.as_slice()).collect();
-        s.serialize_field("content", &content_words)?;
+        s.serialize_field("content", &record.content.words.as_slice())?;
         s.end()
     }
 
     pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<CogRecord, D::Error> {
         #[derive(Deserialize)]
         struct Raw {
             meta: Vec<u64>,
-            content: Vec<Vec<u64>>,
+            content: Vec<u64>,
         }
 
         let raw = Raw::deserialize(deserializer)?;
@@ -60,17 +58,10 @@ mod cogrecord_serde {
             meta.words[i] = w;
         }
 
-        let content = raw
-            .content
-            .iter()
-            .map(|words| {
-                let mut c = Container::zero();
-                for (i, &w) in words.iter().take(CONTAINER_WORDS).enumerate() {
-                    c.words[i] = w;
-                }
-                c
-            })
-            .collect();
+        let mut content = Container::zero();
+        for (i, &w) in raw.content.iter().take(CONTAINER_WORDS).enumerate() {
+            content.words[i] = w;
+        }
 
         Ok(CogRecord { meta, content })
     }

crates/ladybug-contract/src/legacy.rs

@@ -237,7 +237,7 @@ impl From<&V1UnifiedStep> for CogRecord {
             "reasoning": step.reasoning,
         });
         let bytes = serde_json::to_vec(&json).unwrap_or_default();
-        record.content[0] = content_from_bytes(&bytes);
+        record.content = content_from_bytes(&bytes);
 
         record
     }
@@ -281,7 +281,7 @@ impl From<&V1DataEnvelope> for CogRecord {
             // serde envelope until container format is upgraded).
         }
         let bytes = serde_json::to_vec(&envelope.data).unwrap_or_default();
-        record.content[0] = content_from_bytes(&bytes);
+        record.content = content_from_bytes(&bytes);
         record
     }
 }
@@ -337,7 +337,7 @@ impl From<&V1LadybugEnvelope> for CogRecord {
             }
         }
         let bytes = serde_json::to_vec(&envelope.content).unwrap_or_default();
-        record.content[0] = content_from_bytes(&bytes);
+        record.content = content_from_bytes(&bytes);
         record
     }
 }