remove Category from the type system

Category was a manually-assigned label with no remaining functional
purpose (decay was the only behavior it drove, and that's gone).
Remove the enum, its methods, category_counts, the --category search
filter, and all category display. The field remains in the capnp
schema for backwards compatibility but is no longer read or written.

Status and health reports now show NodeType breakdown (semantic,
episodic, daily, weekly, monthly) instead of categories.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/graph.rs

@@ -625,8 +625,18 @@ pub fn health_report(graph: &Graph, store: &Store) -> String {
         }
     }
 
-    // Category breakdown
-    let cats = store.category_counts();
+    // NodeType breakdown
+    let mut type_counts: std::collections::HashMap<&str, usize> = std::collections::HashMap::new();
+    for node in store.nodes.values() {
+        let label = match node.node_type {
+            crate::store::NodeType::EpisodicSession => "episodic",
+            crate::store::NodeType::EpisodicDaily => "daily",
+            crate::store::NodeType::EpisodicWeekly => "weekly",
+            crate::store::NodeType::EpisodicMonthly => "monthly",
+            crate::store::NodeType::Semantic => "semantic",
+        };
+        *type_counts.entry(label).or_default() += 1;
+    }
 
     // Load history for deltas
     let history = load_metrics_history();
@@ -665,16 +675,16 @@ Power-law α: {alpha:.2}{alpha_d} (2=hub-dominated, 3=healthy, >3=egalitarian)
 Degree Gini: {gini:.3}{gini_d} (0=equal, 1=one-hub)
 
 Community sizes (top 5): {top5}
-Categories: core={core} tech={tech} gen={gen} obs={obs} task={task}",
+Types: semantic={semantic} episodic={episodic} daily={daily} weekly={weekly} monthly={monthly}",
         top5 = sizes.iter().take(5)
             .map(|s| s.to_string())
             .collect::<Vec<_>>()
             .join(", "),
-        core = cats.get("core").unwrap_or(&0),
-        tech = cats.get("tech").unwrap_or(&0),
-        gen = cats.get("gen").unwrap_or(&0),
-        obs = cats.get("obs").unwrap_or(&0),
-        task = cats.get("task").unwrap_or(&0),
+        semantic = type_counts.get("semantic").unwrap_or(&0),
+        episodic = type_counts.get("episodic").unwrap_or(&0),
+        daily = type_counts.get("daily").unwrap_or(&0),
+        weekly = type_counts.get("weekly").unwrap_or(&0),
+        monthly = type_counts.get("monthly").unwrap_or(&0),
     );
 
     // Orphan edges