Phase 2b (0.6.1-beta): dual-write file_holders on all propagation events

Populate the flat holder set alongside every existing post_upstream /
post_downstream / blob_upstream / blob_downstream write so that read
paths can be switched over in the next commit without losing continuity.

Events wired:
- Pull sync receive (3 paths in connection.rs)
- PostPush receive (public posts only after Phase 1)
- PostFetch via notification (discovery pull)
- PostDownstreamRegister
- Replication accept (downstream) + replication-driven pull (upstream)
- Attachment upstream recorded after replication blob fetch
- ManifestPush receive (remote is a CID holder)
- ManifestPush send (downstream peer becomes CID holder)
- Blob fetch fallback (upstream lateral sources)

Direction is tracked as Received vs Sent. Not load-bearing for routing;
retained for future use. LRU cap of 5 enforced on every touch.

Legacy upstream/downstream writes remain in place; they'll go away
together with the table drops at the end of this phase.

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

crates/core/src/node.rs

@@ -1351,6 +1351,12 @@ impl Node {
                 .map(|m| m.host_addresses.clone())
                 .unwrap_or_default();
             let _ = storage.store_blob_upstream(cid, from_peer, &source_addrs);
+            let _ = storage.touch_file_holder(
+                cid,
+                from_peer,
+                &source_addrs,
+                crate::storage::HolderDirection::Received,
+            );
         }
         Ok(data)
     }
@@ -1413,6 +1419,12 @@ impl Node {
                         }
                     }
                     let _ = storage.store_blob_upstream(cid, &lateral, &[]);
+                    let _ = storage.touch_file_holder(
+                        cid,
+                        &lateral,
+                        &[],
+                        crate::storage::HolderDirection::Received,
+                    );
                     return Ok(Some(data));
                 }
                 Ok((None, response)) => {