Add stored-query registry loader and GraphHandle wiring

Add a `queries` module: QueryRegistry loads each declared `.gq` entry,
parses it, and selects the query whose symbol matches the manifest key,
asserting the two agree (key == `query <name>` symbol). Identity is the
query name; a key/symbol mismatch is a load-time error. Errors are
collected, not fail-fast, so a bad registry surfaces every broken entry
at once. Schema type-checking is deliberately left to a separate pass so
the loader stays callable without an open engine.

Thread an `Option<Arc<QueryRegistry>>` through GraphHandle alongside the
per-graph policy; the URI-canonicalizing clone propagates it. Production
openers default to None for now — the boot path loads and attaches the
registry in a later change.

- QueryRegistry::{from_specs, load, lookup, iter}; StoredQuery::is_mutation
- GraphHandle.queries field, propagated on canonical clone
- registry unit tests: identity match/mismatch, multi-query selection,
  per-entry parse errors, error collection, mutation classification

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/omnigraph-server/tests/openapi.rs

@@ -1117,6 +1117,7 @@ async fn app_for_multi_mode(graph_ids: &[&str]) -> (Vec<tempfile::TempDir>, Rout
             uri: graph_uri,
             engine: Arc::new(engine),
             policy: None,
+            queries: None,
         }));
         dirs.push(dir);
     }

crates/omnigraph-server/tests/server.rs

@@ -4690,6 +4690,7 @@ mod multi_graph_startup {
                 uri: graph_uri,
                 engine: Arc::new(engine),
                 policy: None,
+                queries: None,
             }));
             dirs.push(dir);
         }
@@ -4985,12 +4986,14 @@ graphs:
             uri: graph_uri.clone(),
             engine: Arc::clone(&engine),
             policy: None,
+            queries: None,
         });
         let beta = Arc::new(GraphHandle {
             key: GraphKey::cluster(GraphId::try_from("beta").unwrap()),
             uri: format!("file://{graph_uri}/"),
             engine,
             policy: None,
+            queries: None,
         });
 
         match GraphRegistry::from_handles(vec![alpha, beta]) {
@@ -5016,6 +5019,7 @@ graphs:
             uri: format!("file://{graph_uri}/"),
             engine: Arc::new(engine),
             policy: None,
+            queries: None,
         });
 
         let registry = GraphRegistry::from_handles(vec![handle]).unwrap();
@@ -5383,6 +5387,7 @@ graphs:
             uri: graph_uri,
             engine: Arc::new(engine),
             policy: None,
+            queries: None,
         });
         let tokens = vec![("act-andrew".to_string(), "secret-token".to_string())];
         let workload = omnigraph_server::workload::WorkloadController::from_env();
@@ -5450,6 +5455,7 @@ graphs:
                 uri: graph_uri,
                 engine: Arc::new(engine),
                 policy: None,
+                queries: None,
             }));
         }