feat(mesh): ContentRef typed variant + send/fetch RPCs (Phase 3b)

Adds attachment sharing over the mesh: a ContentRef envelope (variant 19)
carries the blob CID, size, mime, optional thumb/caption, and a per-peer
HMAC capability URL so the recipient fetches the full blob out-of-band via
`GET {sender_onion}/blob/{cid}?cap=..&exp=..&peer=..`. BlobStore is shared
from ApiHandler into RpcHandler so mesh.send-content and mesh.fetch-content
(reqwest via TOR_SOCKS_PROXY) hit the same store and cap_key.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/mod.rs

@@ -71,6 +71,13 @@ pub struct RpcHandler {
     response_cache: ResponseCache,
     mesh_service: Arc<tokio::sync::RwLock<Option<crate::mesh::MeshService>>>,
     transport_router: Arc<tokio::sync::RwLock<Option<Arc<crate::transport::TransportRouter>>>>,
+    /// Shared content-addressed blob store. Set by ApiHandler after construction
+    /// so mesh.send-content / mesh.fetch-content RPCs can reach it without a
+    /// second instance and duplicated cap_key.
+    pub(crate) blob_store: Arc<tokio::sync::RwLock<Option<Arc<crate::blobs::BlobStore>>>>,
+    /// Our own Ed25519 pubkey hex — needed by ContentRef senders for cap scoping
+    /// and by ContentRef receivers to request caps scoped to themselves.
+    pub(crate) self_pubkey_hex: Arc<tokio::sync::RwLock<Option<String>>>,
 }
 
 impl RpcHandler {
@@ -128,6 +135,8 @@ impl RpcHandler {
             response_cache: ResponseCache::new(5),
             mesh_service: Arc::new(tokio::sync::RwLock::new(None)),
             transport_router: Arc::new(tokio::sync::RwLock::new(None)),
+            blob_store: Arc::new(tokio::sync::RwLock::new(None)),
+            self_pubkey_hex: Arc::new(tokio::sync::RwLock::new(None)),
         })
     }
 
@@ -141,6 +150,13 @@ impl RpcHandler {
         *self.transport_router.write().await = Some(router);
     }
 
+    /// Share the blob store + our pubkey so mesh.send-content / fetch-content
+    /// can reach them. Called once from ApiHandler::new.
+    pub async fn set_blob_store(&self, store: Arc<crate::blobs::BlobStore>, self_pubkey_hex: String) {
+        *self.blob_store.write().await = Some(store);
+        *self.self_pubkey_hex.write().await = Some(self_pubkey_hex);
+    }
+
     /// Get reference to the mesh service Arc (for MeshTransport wrapper).
     pub fn mesh_service_arc(&self) -> Arc<tokio::sync::RwLock<Option<crate::mesh::MeshService>>> {
         Arc::clone(&self.mesh_service)