eliminate schema_fit: it's clustering coefficient

schema_fit was algebraically identical to clustering_coefficient
(both compute 2E/(d*(d-1)) = fraction of connected neighbor pairs).
Remove the redundant function, field, and metrics column.

- Delete schema_fit() and schema_fit_all() from graph.rs
- Remove schema_fit field from Node struct
- Remove avg_schema_fit from MetricsSnapshot (duplicated avg_cc)
- Replace all callers with graph.clustering_coefficient()
- Rename ReplayItem.schema_fit to .cc
- Query: "cc" and "schema_fit" both resolve from graph CC
- Low-CC count folded into health report CC line

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/query.rs

@@ -184,12 +184,9 @@ fn resolve_field(field: &str, key: &str, store: &Store, graph: &Graph) -> Option
         "community_id"  => {
             graph.communities().get(key).map(|&c| Value::Num(c as f64))
         }
-        "clustering_coefficient" => {
+        "clustering_coefficient" | "schema_fit" | "cc" => {
             Some(Value::Num(graph.clustering_coefficient(key) as f64))
         }
-        "schema_fit"    => {
-            node.schema_fit.map(|f| Value::Num(f as f64))
-        }
         _ => None,
     }
 }