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/handler/mod.rs

@@ -78,6 +78,13 @@ impl ApiHandler {
         let blob_store = Arc::new(BlobStore::open(&config.data_dir, cap_key).await?);
         let self_pubkey_hex = hex::encode(identity.signing_key().verifying_key().as_bytes());
 
+        // Share blob store with the RPC layer so mesh.send-content /
+        // mesh.fetch-content can reach the same instance (single cap_key,
+        // single on-disk root) without re-opening it.
+        rpc_handler
+            .set_blob_store(blob_store.clone(), self_pubkey_hex.clone())
+            .await;
+
         Ok(Self {
             config,
             rpc_handler,