perf(engine): reuse the embedding client across queries (RFC-012 Phase 2)

Hold the resolved EmbeddingClient in an Arc<tokio::sync::OnceCell> on the Omnigraph handle, built lazily on the first nearest($v, "string") that needs embedding (so a graph that never embeds needs no provider key) and reused by every later query — dropping the per-query EmbeddingClient::from_env() rebuild and keeping the provider connection pool warm. The cell is threaded through execute_query -> extract_search_mode/extract_sub_search_mode -> resolve_nearest_query_vec via a pub(crate) embedding_cell() accessor (the field is module-private). Covered by the string-nearest paths in tests/search.rs (direct, literal, RRF).

crates/omnigraph/src/db/omnigraph.rs

@@ -156,6 +156,12 @@ pub struct Omnigraph {
     /// `apply_schema_as` consults this field (PR #2 proof-of-concept);
     /// PR #3 fans the `enforce()` call out to the remaining writers.
     policy: Option<Arc<dyn omnigraph_policy::PolicyChecker>>,
+    /// Lazily-built, reused-across-queries embedding client. Built on the first
+    /// `nearest($v, "string")` that needs server-side embedding (so a graph that
+    /// never embeds needs no provider key), then shared by every later query —
+    /// avoids the per-query `from_env()` rebuild and keeps the provider HTTP
+    /// connection pool warm. `OnceCell` guarantees a single initialization.
+    embedding: Arc<tokio::sync::OnceCell<crate::embedding::EmbeddingClient>>,
 }
 
 /// Whether [`Omnigraph::open`] runs the open-time recovery sweep.
@@ -319,6 +325,7 @@ impl Omnigraph {
             write_queue: Arc::new(crate::db::write_queue::WriteQueueManager::new()),
             merge_exclusive: Arc::new(tokio::sync::Mutex::new(())),
             policy: None,
+            embedding: Arc::new(tokio::sync::OnceCell::new()),
         })
     }
 
@@ -418,6 +425,7 @@ impl Omnigraph {
             write_queue: Arc::new(crate::db::write_queue::WriteQueueManager::new()),
             merge_exclusive: Arc::new(tokio::sync::Mutex::new(())),
             policy: None,
+            embedding: Arc::new(tokio::sync::OnceCell::new()),
         })
     }
 
@@ -465,6 +473,15 @@ impl Omnigraph {
         self
     }
 
+    /// The lazily-initialized, reused-across-queries embedding client cell
+    /// (see the `embedding` field doc). The query executor resolves the client
+    /// through this on the first `nearest($v, "string")` that needs embedding.
+    pub(crate) fn embedding_cell(
+        &self,
+    ) -> &tokio::sync::OnceCell<crate::embedding::EmbeddingClient> {
+        &self.embedding
+    }
+
     /// Engine-layer policy enforcement gate (MR-722 chassis core).
     ///
     /// * If no policy is installed → no-op (returns `Ok(())`).

crates/omnigraph/src/exec/query.rs

@@ -31,7 +31,15 @@ impl Omnigraph {
             GraphIndexHandle::none()
         };
 
-        execute_query(&ir, params, &resolved.snapshot, &graph_index, &catalog).await
+        execute_query(
+            &ir,
+            params,
+            &resolved.snapshot,
+            &graph_index,
+            &catalog,
+            self.embedding_cell(),
+        )
+        .await
     }
 
     /// Run a named query against the graph as it existed at a prior manifest version.
@@ -72,7 +80,15 @@ impl Omnigraph {
             GraphIndexHandle::none()
         };
 
-        execute_query(&ir, params, &snapshot, &graph_index, &catalog).await
+        execute_query(
+            &ir,
+            params,
+            &snapshot,
+            &graph_index,
+            &catalog,
+            self.embedding_cell(),
+        )
+        .await
     }
 }
 
@@ -102,6 +118,7 @@ async fn extract_search_mode(
     ir: &QueryIR,
     params: &ParamMap,
     catalog: &Catalog,
+    embedding: &tokio::sync::OnceCell<EmbeddingClient>,
 ) -> Result<SearchMode> {
     if ir.order_by.is_empty() {
         return Ok(SearchMode::default());
@@ -114,7 +131,8 @@ async fn extract_search_mode(
             query,
         } => {
             let vec =
-                resolve_nearest_query_vec(ir, catalog, variable, property, query, params).await?;
+                resolve_nearest_query_vec(ir, catalog, variable, property, query, params, embedding)
+                    .await?;
             let k = ir.limit.ok_or_else(|| {
                 OmniError::manifest("nearest() ordering requires a limit clause".to_string())
             })? as usize;
@@ -157,9 +175,10 @@ async fn extract_search_mode(
                 .unwrap_or(60) as u32;
 
             let primary_mode =
-                extract_sub_search_mode(ir, primary, params, catalog, ir.limit).await?;
+                extract_sub_search_mode(ir, primary, params, catalog, ir.limit, embedding).await?;
             let secondary_mode =
-                extract_sub_search_mode(ir, secondary, params, catalog, ir.limit).await?;
+                extract_sub_search_mode(ir, secondary, params, catalog, ir.limit, embedding)
+                    .await?;
 
             Ok(SearchMode {
                 rrf: Some(RrfMode {
@@ -182,6 +201,7 @@ async fn extract_sub_search_mode(
     params: &ParamMap,
     catalog: &Catalog,
     limit: Option<u64>,
+    embedding: &tokio::sync::OnceCell<EmbeddingClient>,
 ) -> Result<SearchMode> {
     match expr {
         IRExpr::Nearest {
@@ -190,7 +210,8 @@ async fn extract_sub_search_mode(
             query,
         } => {
             let vec =
-                resolve_nearest_query_vec(ir, catalog, variable, property, query, params).await?;
+                resolve_nearest_query_vec(ir, catalog, variable, property, query, params, embedding)
+                    .await?;
             let k = limit.unwrap_or(100) as usize;
             Ok(SearchMode {
                 nearest: Some((variable.clone(), property.clone(), vec, k)),
@@ -229,15 +250,20 @@ async fn resolve_nearest_query_vec(
     property: &str,
     expr: &IRExpr,
     params: &ParamMap,
+    embedding: &tokio::sync::OnceCell<EmbeddingClient>,
 ) -> Result<Vec<f32>> {
     let lit = resolve_literal_or_param(expr, params)?;
     match lit {
         Literal::List(_) => literal_to_f32_vec(&lit),
         Literal::String(text) => {
             let expected_dim = nearest_property_dimension(ir, catalog, variable, property)?;
-            EmbeddingClient::from_env()?
-                .embed_query_text(&text, expected_dim)
-                .await
+            // Lazily resolve the per-handle client once, then reuse it across
+            // queries (keeps the provider connection pool warm); a graph that
+            // never embeds never builds a client and needs no provider key.
+            let client = embedding
+                .get_or_try_init(|| async { EmbeddingClient::from_env() })
+                .await?;
+            client.embed_query_text(&text, expected_dim).await
         }
         _ => Err(OmniError::manifest(
             "nearest query must be a string or list of floats".to_string(),
@@ -341,8 +367,9 @@ pub async fn execute_query(
     snapshot: &Snapshot,
     graph_index: &GraphIndexHandle<'_>,
     catalog: &Catalog,
+    embedding: &tokio::sync::OnceCell<EmbeddingClient>,
 ) -> Result<QueryResult> {
-    let search_mode = extract_search_mode(ir, params, catalog).await?;
+    let search_mode = extract_search_mode(ir, params, catalog, embedding).await?;
 
     // RRF requires forked execution
     if let Some(ref rrf) = search_mode.rrf {