chore(clippy): close len_zero + await_holding_lock blockers

Two clippy errors surfacing on the lance-cache feature gate:

1. lance_cache.rs:110 (mine, from the versioning PR step) — `arr.len() > 0`
   → `!arr.is_empty()`.

2. registry.rs:185 (pre-existing) — `MutexGuard` held across an await
   point. PR #369's body claimed this was fixed but it landed unfixed.
   Clone `last_root_checksum` out of the read guard before the
   `set_last_root_checksum().await` call so the guard drops first.

`cargo clippy -p lance-graph-ontology --features lance-cache --tests
--no-deps -- -D warnings` exits 0 after these two.

Author: claude

crates/lance-graph-ontology/src/lance_cache.rs

@@ -107,7 +107,7 @@ impl LanceWriter {
             let Some(arr) = col.as_any().downcast_ref::<UInt32Array>() else {
                 return Ok(None);
             };
-            if arr.len() > 0 {
+            if !arr.is_empty() {
                 return Ok(Some(arr.value(0)));
             }
         }

crates/lance-graph-ontology/src/registry.rs

@@ -182,8 +182,11 @@ impl OntologyRegistry {
             let writer = LanceWriter::open_or_create(lance_path).await?;
             let rows: Vec<MappingRow> = self.inner.read().unwrap().rows.clone();
             writer.flush(&rows).await?;
-            if let Some(cs) = &self.inner.read().unwrap().last_root_checksum {
-                writer.set_last_root_checksum(cs).await?;
+            // Clone the checksum out of the read guard before the await so
+            // clippy::await_holding_lock stays green.
+            let last_checksum = self.inner.read().unwrap().last_root_checksum.clone();
+            if let Some(cs) = last_checksum {
+                writer.set_last_root_checksum(&cs).await?;
             }
         }
         Ok(report)