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/capnp_store.rs

@@ -145,8 +145,6 @@ pub struct Node {
     #[serde(default)]
     pub clustering_coefficient: Option<f32>,
     #[serde(default)]
-    pub schema_fit: Option<f32>,
-    #[serde(default)]
     pub degree: Option<u32>,
 }
 
@@ -925,7 +923,6 @@ impl Store {
             position: 0,
             community_id: None,
             clustering_coefficient: None,
-            schema_fit: None,
             degree: None,
         }
     }
@@ -1505,13 +1502,11 @@ impl Store {
     pub fn update_graph_metrics(&mut self) {
         let g = self.build_graph();
         let communities = g.communities();
-        let fits = graph::schema_fit_all(&g);
 
         for (key, node) in &mut self.nodes {
             node.community_id = communities.get(key).copied();
             node.clustering_coefficient = Some(g.clustering_coefficient(key));
             node.degree = Some(g.degree(key) as u32);
-            node.schema_fit = fits.get(key).copied();
         }
     }
 
@@ -1874,7 +1869,6 @@ fn read_content_node(r: memory_capnp::content_node::Reader) -> Result<Node, Stri
         position: r.get_position(),
         community_id: None,
         clustering_coefficient: None,
-        schema_fit: None,
         degree: None,
     })
 }