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);
     }