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/src/lib.rs

@@ -4,6 +4,7 @@ pub mod config;
 pub mod graph_id;
 pub mod identity;
 pub mod policy;
+pub mod queries;
 pub mod registry;
 pub mod workload;
 
@@ -436,6 +437,9 @@ impl AppState {
             uri,
             engine: Arc::new(db),
             policy: policy_engine,
+            // Stored-query registry is wired in by the boot path;
+            // single-mode construction defaults to "no stored queries".
+            queries: None,
         });
         Self {
             routing: GraphRouting::Single { handle },
@@ -1146,6 +1150,8 @@ async fn open_single_graph(cfg: GraphStartupConfig) -> Result<Arc<GraphHandle>>
         uri,
         engine: Arc::new(db),
         policy: policy_arc,
+        // Stored-query registry is loaded + checked by the boot path.
+        queries: None,
     }))
 }