fix: metadata-only update() never rewrites the noun record — the unconditional whole-vector save turned per-entity stat touches into full rewrites+fsync, amplifying read-heavy sweeps into disk saturation on a production deployment

Also: idle PathResolver stats tick no longer logs NaN% every minute (logs
only on new traffic, via prodLog); graph-lsm-* key family recognized as
system resources (kills the per-boot unknown-key warning on provider-backed
brains). Four regression pins in tests/integration/update-write-granularity.

src/brainy.ts

@@ -3161,18 +3161,23 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           new UpdateNounMetadataOperation(this.storage, params.id, updatedMetadata)
         )
 
-        // Operation 2: Update vector data (will use updated type cache)
-        tx.addOperation(
-          new SaveNounOperation(this.storage, {
-            id: params.id,
-            vector,
-            connections: new Map(),
-            level: 0
-          })
-        )
-
-        // Operation 3-4: Update HNSW index (remove and re-add if reindexing needed)
+        // Operations 2-4: vector-record write + HNSW reindex — ONLY when the
+        // vector side actually changed (new data/vector/type). A metadata-only
+        // update must never rewrite the noun record: the record carries the
+        // full vector, so an unconditional save turned every metadata touch
+        // into a whole-vector rewrite + fsync — under a read-heavy consumer
+        // sweep that bumps per-entity stats, this amplified into disk
+        // saturation on a production deployment (SELF-ENGINE-RESTART-GRIND,
+        // 2026-07-29: 5.8GB written in 40min from ~50 recalls/min).
         if (needsReindexing) {
+          tx.addOperation(
+            new SaveNounOperation(this.storage, {
+              id: params.id,
+              vector,
+              connections: new Map(),
+              level: 0
+            })
+          )
           tx.addOperation(
             new RemoveFromVectorIndexOperation(this.index, params.id, existing.vector)
           )