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/migrate.rs

@@ -221,7 +221,6 @@ pub fn migrate() -> Result<(), String> {
             position: 0,
             community_id: None,
             clustering_coefficient: None,
-            schema_fit: None,
             degree: None,
         };
         all_nodes.push(node);
@@ -261,7 +260,6 @@ pub fn migrate() -> Result<(), String> {
             position: 0,
             community_id: None,
             clustering_coefficient: None,
-            schema_fit: None,
             degree: None,
         };
         all_nodes.push(node);