fix: aggregation surfaces materialize/state-load failures loudly

The debounced materialization caught its failure with an empty `catch (() => {})`,
silently leaving the materialized Measurement entity stale; the aggregation-index
init() state-load failure was swallowed the same way, leaving aggregates reading
empty with no signal. Both are non-fatal (values rebuild via backfill-on-query),
but a silent stale/empty read violates loud-errors-never-quiet-losses. Both now
emit a loud warning naming the affected group / cause.

src/aggregation/materializer.ts

@@ -14,6 +14,7 @@ import type {
   AggregateMetricDef
 } from '../types/brainy.types.js'
 import { serializeGroupKey } from './AggregationIndex.js'
+import { prodLog } from '../utils/logger.js'
 
 /**
  * Callback interface for the materializer to create/update Brainy entities.
@@ -86,8 +87,15 @@ export class AggregateMaterializer {
     if (existing) clearTimeout(existing)
 
     this.debounceTimers.set(key, setTimeout(() => {
-      this.materializeOne(key).catch(() => {
-        // Non-fatal — materialization is derived data
+      this.materializeOne(key).catch((err) => {
+        // Materialization is derived data (rebuildable via backfill-on-query), so
+        // a failure is non-fatal — but it leaves the materialized Measurement
+        // entity STALE. Surface it loudly rather than swallow it silently.
+        prodLog.warn(
+          `[Aggregation] Failed to materialize aggregate group '${key}': ` +
+          `${(err as Error).message}. The materialized value is stale until the ` +
+          `next successful materialization or a backfill-on-query.`
+        )
       })
     }, debounceMs))
   }

tests/unit/aggregation/materialize-failure-loud.test.ts

@@ -0,0 +1,51 @@
+/**
+ * @module tests/unit/aggregation/materialize-failure-loud
+ * @description Aggregation (Pattern C): a failed materialization must not be
+ * swallowed. `scheduleMaterialize`'s debounced `materializeOne(key).catch(() => {})`
+ * silently discarded the error, leaving the materialized Measurement entity stale
+ * with no signal. It now emits a loud warning (the value is still rebuildable via
+ * backfill-on-query, so it stays non-fatal — but visible). Load-bearing assertion:
+ * prodLog.warn fires instead of an empty catch.
+ */
+import { describe, it, expect, vi, afterEach } from 'vitest'
+import { AggregateMaterializer } from '../../../src/aggregation/materializer.js'
+import { prodLog } from '../../../src/utils/logger.js'
+import { NounType } from '../../../src/types/graphTypes.js'
+
+describe('Aggregation — swallowed materialize failure is surfaced', () => {
+  afterEach(() => {
+    vi.restoreAllMocks()
+    vi.useRealTimers()
+  })
+
+  it('logs loudly when materializeOne rejects', async () => {
+    vi.useFakeTimers()
+    const warn = vi.spyOn(prodLog, 'warn').mockImplementation(() => {})
+
+    // Brain access whose add()/update() always reject → doMaterialize throws.
+    const brain = {
+      add: vi.fn().mockRejectedValue(new Error('storage down')),
+      update: vi.fn().mockRejectedValue(new Error('storage down'))
+    }
+    const m = new AggregateMaterializer(brain as any, 10)
+    const def: any = {
+      name: 'sales',
+      source: { type: NounType.Document },
+      groupBy: ['region'],
+      metrics: { total: { op: 'count' } },
+      materialize: { debounceMs: 10 }
+    }
+    m.scheduleMaterialize(
+      'sales',
+      def,
+      { region: 'us' },
+      { groupKey: { region: 'us' }, metrics: { total: { count: 1 } } } as any
+    )
+
+    await vi.advanceTimersByTimeAsync(20) // fire the debounce timer
+    await Promise.resolve()
+
+    expect(warn).toHaveBeenCalled()
+    expect(String(warn.mock.calls[0][0])).toMatch(/Failed to materialize aggregate group/)
+  })
+})