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.

tests/integration/update-write-granularity.test.ts

@@ -0,0 +1,126 @@
+/**
+ * @module tests/integration/update-write-granularity
+ * @description Write-granularity law for update() (SELF-ENGINE-RESTART-GRIND,
+ * 2026-07-29): a metadata-only update must NEVER rewrite the noun record —
+ * the record carries the full vector, so an unconditional save turns every
+ * metadata touch into a whole-vector rewrite + fsync. Under a read-heavy
+ * consumer sweep bumping per-entity stats this amplified into disk saturation
+ * on a production deployment. Laws:
+ * (1) metadata-only update() → zero saveNoun calls (metadata leg only);
+ * (2) data/vector/type-changing update() → saveNoun runs (the vector leg and
+ *     HNSW reindex still happen when the vector side actually changed);
+ * (3) the metadata-only path still lands: merged metadata readable, _rev
+ *     bumped, find() by the new field sees the entity.
+ */
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { Brainy } from '../../src/brainy.js'
+import { NounType } from '../../src/types/graphTypes.js'
+
+const stubEmbedding = async (text: string): Promise<number[]> => {
+  const hash = text.split('').reduce((acc, char) => acc + char.charCodeAt(0), 0)
+  return new Array(384).fill(0).map((_, i) => Math.sin(hash + i))
+}
+
+describe('update() write granularity', () => {
+  let brain: Brainy
+
+  beforeEach(async () => {
+    brain = new Brainy({
+      requireSubtype: false,
+      storage: { type: 'memory' as const },
+      embeddingFunction: stubEmbedding
+    })
+    await brain.init()
+  })
+
+  afterEach(async () => {
+    await brain.close()
+  })
+
+  it('metadata-only update never rewrites the noun record (no vector rewrite)', async () => {
+    const id = await brain.add({
+      data: 'granularity law subject',
+      type: NounType.Concept,
+      metadata: { touched: 0 }
+    })
+
+    const storage = (brain as any).storage
+    const saveNounSpy = vi.spyOn(storage, 'saveNoun')
+
+    await brain.update({ id, metadata: { touched: 1 } })
+
+    expect(saveNounSpy).not.toHaveBeenCalled()
+    saveNounSpy.mockRestore()
+
+    // The metadata leg still landed with full semantics.
+    const after = await brain.get(id, { includeVectors: true })
+    expect(after?.metadata?.touched).toBe(1)
+    expect(after?._rev).toBe(2)
+    expect(Array.isArray(after?.vector) && after!.vector!.length).toBe(384)
+
+    const found = await brain.find({ where: { touched: 1 } })
+    expect(found.some((r: any) => r.id === id)).toBe(true)
+  })
+
+  it('confidence/weight/subtype-only updates also skip the noun record', async () => {
+    const id = await brain.add({
+      data: 'reserved-field touch subject',
+      type: NounType.Concept,
+      metadata: {}
+    })
+
+    const storage = (brain as any).storage
+    const saveNounSpy = vi.spyOn(storage, 'saveNoun')
+
+    await brain.update({ id, confidence: 0.5, weight: 2, subtype: 'note' })
+
+    expect(saveNounSpy).not.toHaveBeenCalled()
+    saveNounSpy.mockRestore()
+
+    const after = await brain.get(id)
+    expect(after?.confidence).toBe(0.5)
+    expect(after?.subtype).toBe('note')
+  })
+
+  it('data-changing update still writes the noun record and reindexes', async () => {
+    const id = await brain.add({
+      data: 'original embedded text',
+      type: NounType.Concept,
+      metadata: {}
+    })
+
+    const before = await brain.get(id, { includeVectors: true })
+
+    const storage = (brain as any).storage
+    const saveNounSpy = vi.spyOn(storage, 'saveNoun')
+
+    await brain.update({ id, data: 'completely different embedded text' })
+
+    expect(saveNounSpy).toHaveBeenCalled()
+    saveNounSpy.mockRestore()
+
+    const after = await brain.get(id, { includeVectors: true })
+    expect(after?.data).toBe('completely different embedded text')
+    expect(after?.vector).not.toEqual(before?.vector)
+  })
+
+  it('explicit-vector update still writes the noun record', async () => {
+    const id = await brain.add({
+      data: 'vector swap subject',
+      type: NounType.Concept,
+      metadata: {}
+    })
+
+    const storage = (brain as any).storage
+    const saveNounSpy = vi.spyOn(storage, 'saveNoun')
+
+    const newVector = new Array(384).fill(0).map((_, i) => Math.cos(i))
+    await brain.update({ id, vector: newVector })
+
+    expect(saveNounSpy).toHaveBeenCalled()
+    saveNounSpy.mockRestore()
+
+    const after = await brain.get(id, { includeVectors: true })
+    expect(after?.vector?.[0]).toBeCloseTo(1) // cos(0)
+  })
+})